I have MediaTek 7620N-based router and have built a custom image for it, built from Chaos Calmer 15.05 sources with xl2tpd package responsible for L2TP connections. When connecting to Mikrotik server with router as a client, L2TP works fine, and shows following log on startup:

Fri Nov 17 06:24:26 2017 daemon.info pppd[24303]: Plugin pppol2tp.so loaded.
Fri Nov 17 06:24:26 2017 daemon.notice pppd[24303]: pppd 2.4.7 started by root, uid 0
Fri Nov 17 06:24:26 2017 daemon.debug pppd[24303]: using channel 106
Fri Nov 17 06:24:26 2017 kern.info kernel: [ 8805.760000] l2tp-vpn: renamed from ppp0
Fri Nov 17 06:24:26 2017 daemon.info pppd[24303]: Using interface l2tp-vpn
Fri Nov 17 06:24:26 2017 daemon.notice pppd[24303]: Connect: l2tp-vpn <--> 
Fri Nov 17 06:24:26 2017 daemon.debug pppd[24303]: PPPoL2TP options: debugmask 0
Fri Nov 17 06:24:26 2017 daemon.debug pppd[24303]: sent [LCP ConfReq id=0x1 <asyncmap 0x0> <magic 0x7872e558>]
Fri Nov 17 06:24:26 2017 daemon.debug pppd[24303]: rcvd [LCP ConfRej id=0x1 <asyncmap 0x0>]
Fri Nov 17 06:24:26 2017 daemon.debug pppd[24303]: sent [LCP ConfReq id=0x2 <magic 0x7872e558>]
Fri Nov 17 06:24:26 2017 daemon.debug pppd[24303]: rcvd [LCP ConfAck id=0x2 <magic 0x7872e558>]
Fri Nov 17 06:24:26 2017 daemon.debug pppd[24303]: rcvd [LCP ConfReq id=0x2 <auth chap MS-v2> <mru 1462> <magic 0xdc900d23>]
Fri Nov 17 06:24:26 2017 daemon.debug pppd[24303]: sent [LCP ConfAck id=0x2 <auth chap MS-v2> <mru 1462> <magic 0xdc900d23>]
Fri Nov 17 06:24:26 2017 daemon.debug pppd[24303]: PPPoL2TP options: debugmask 0
Fri Nov 17 06:24:26 2017 daemon.debug pppd[24303]: sent [LCP EchoReq id=0x0 magic=0x7872e558]
Fri Nov 17 06:24:26 2017 daemon.debug pppd[24303]: rcvd [CHAP Challenge id=0x1 <28bdc5fc67b38242f821425b4b94a422>, name = "MikroTik"]
Fri Nov 17 06:24:27 2017 daemon.debug pppd[24303]: added response cache entry 0
Fri Nov 17 06:24:27 2017 daemon.debug pppd[24303]: sent [CHAP Response id=0x1 <8722d010e077633dfe80a3131f19401200000000000000001124688346fc4ed8b1b57897e7da7becf2892061a29e72e800>, name = "admin"]
Fri Nov 17 06:24:27 2017 daemon.debug pppd[24303]: rcvd [LCP EchoRep id=0x0 magic=0xdc900d23]
Fri Nov 17 06:24:27 2017 daemon.debug pppd[24303]: rcvd [CHAP Success id=0x1 "S=A48E289301FE5053261061B8088F221A8466A68D"]
Fri Nov 17 06:24:27 2017 daemon.debug pppd[24303]: response found in cache (entry 0)
Fri Nov 17 06:24:27 2017 daemon.notice pppd[24303]: CHAP authentication succeeded
Fri Nov 17 06:24:27 2017 daemon.debug pppd[24303]: sent [CCP ConfReq id=0x1 <mppe +H -M +S -L -D -C>]
Fri Nov 17 06:24:27 2017 daemon.debug pppd[24303]: rcvd [IPCP ConfReq id=0x1 <addr 10.10.0.193>]
Fri Nov 17 06:24:27 2017 daemon.debug pppd[24303]: sent [IPCP TermAck id=0x1]
Fri Nov 17 06:24:27 2017 daemon.debug pppd[24303]: rcvd [proto=0x8281] 01 01 00 04
Fri Nov 17 06:24:27 2017 daemon.warn pppd[24303]: Unsupported protocol 0x8281 received
Fri Nov 17 06:24:27 2017 daemon.debug pppd[24303]: rcvd [IPCP ConfReq id=0x2 <addr 10.10.0.193>]
Fri Nov 17 06:24:27 2017 daemon.debug pppd[24303]: sent [IPCP ConfAck id=0x2 <addr 10.10.0.193>]
Fri Nov 17 06:24:27 2017 daemon.notice pppd[24303]: local  IP address 10.10.0.244
Fri Nov 17 06:24:27 2017 daemon.notice pppd[24303]: remote IP address 10.10.0.193
Fri Nov 17 06:24:27 2017 daemon.notice pppd[24303]: primary   DNS address 8.8.8.8
Fri Nov 17 06:24:27 2017 daemon.notice pppd[24303]: secondary DNS address 109.71.32.10
Fri Nov 17 06:24:27 2017 daemon.debug pppd[24303]: Script /lib/netifd/ppp-up started (pid 24335)
Fri Nov 17 06:24:28 2017 daemon.notice netifd: Network device 'l2tp-vpn' link is up
Fri Nov 17 06:24:28 2017 daemon.notice netifd: Interface 'vpn' is now up
Fri Nov 17 06:24:28 2017 daemon.info dnsmasq[23609]: reading /tmp/resolv.conf.auto
Fri Nov 17 06:24:28 2017 daemon.info dnsmasq[23609]: using local addresses only for domain lan
Fri Nov 17 06:24:28 2017 daemon.info dnsmasq[23609]: using nameserver 8.8.8.8#53
Fri Nov 17 06:24:28 2017 daemon.info dnsmasq[23609]: using nameserver 109.71.32.10#53
Fri Nov 17 06:24:28 2017 daemon.info dnsmasq[23609]: using nameserver 10.10.1.1#53
Fri Nov 17 06:24:28 2017 daemon.debug pppd[24303]: Script /lib/netifd/ppp-up finished (pid 24335), status = 0x0

But then connection is constantly terminating with following messages shown in log:

Fri Nov 17 06:29:13 2017 daemon.debug pppd[25557]: rcvd [proto=0x1ac2] 68 39 cc c0 eb db bb 71 32 b5 88 35 1d 0c 51 b5 71 5a 4f bc 66 56 b7 b3 b0 2c 2d 19 aa 45 04 a4 ...
Fri Nov 17 06:29:13 2017 daemon.warn pppd[25557]: Unsupported protocol 0x1ac2 received
Fri Nov 17 06:29:13 2017 daemon.debug pppd[25557]: sent [LCP ProtRej id=0x4 1a c2 68 39 cc c0 eb db bb 71 32 b5 88 35 1d 0c 51 b5 71 5a 4f bc 66 56 b7 b3 b0 2c 2d 19 aa 45 ...]
Fri Nov 17 06:29:13 2017 daemon.debug pppd[25557]: rcvd [proto=0x7] c5 a1 75 f3 a6 81 d7 fb a7 21 95 6c 81 c9 2c d3 9f 66 46 04 01 98 6b b7 f1 74 c9 1c d0 df 26 f6 ...
Fri Nov 17 06:29:13 2017 daemon.warn pppd[25557]: Unsupported protocol 0x7 received
Fri Nov 17 06:29:13 2017 daemon.debug pppd[25557]: sent [LCP ProtRej id=0x5 00 07 c5 a1 75 f3 a6 81 d7 fb a7 21 95 6c 81 c9 2c d3 9f 66 46 04 01 98 6b b7 f1 74 c9 1c d0 df ...]
Fri Nov 17 06:29:13 2017 daemon.debug pppd[25557]: rcvd [proto=0xf9] 58 fe 81 b5 41 9c f3 1e cb b5 f4 b2 6e f4 8c 00 db 00 bb 14 dc e7 3d 69 d3 3d dc 24 ac 64 af 05 ...
Fri Nov 17 06:29:13 2017 daemon.warn pppd[25557]: Unsupported protocol 0xf9 received
Fri Nov 17 06:29:13 2017 daemon.debug pppd[25557]: sent [LCP ProtRej id=0x6 00 f9 58 fe 81 b5 41 9c f3 1e cb b5 f4 b2 6e f4 8c 00 db 00 bb 14 dc e7 3d 69 d3 3d dc 24 ac 64 ...]
Fri Nov 17 06:29:13 2017 daemon.debug pppd[25557]: rcvd [proto=0x41] f6 eb bf 71 ae 6d 13 ae 38 c7 7b cb 1d e7 76 2e 5a 63 07 e4 ad 1f c5 b0 73 93 83 0b 12 4d b0 79 ...
Fri Nov 17 06:29:13 2017 daemon.warn pppd[25557]: Unsupported protocol 0x41 received
Fri Nov 17 06:29:13 2017 daemon.debug pppd[25557]: sent [LCP ProtRej id=0x7 00 41 f6 eb bf 71 ae 6d 13 ae 38 c7 7b cb 1d e7 76 2e 5a 63 07 e4 ad 1f c5 b0 73 93 83 0b 12 4d ...]
Fri Nov 17 06:29:13 2017 daemon.debug pppd[25557]: rcvd [proto=0x6482] 6c 2d 49 b9 24 64 c9 f9 14 43 d4 cf bd c3 f6 2f 38 bb d0 79 46 9e 92 b9 54 3c 20 f4 c3 5f 90 f4 ...
Fri Nov 17 06:29:13 2017 daemon.warn pppd[25557]: Unsupported protocol 0x6482 received
Fri Nov 17 06:29:13 2017 daemon.debug pppd[25557]: sent [LCP ProtRej id=0x8 64 82 6c 2d 49 b9 24 64 c9 f9 14 43 d4 cf bd c3 f6 2f 38 bb d0 79 46 9e 92 b9 54 3c 20 f4 c3 5f ...]
Fri Nov 17 06:29:13 2017 daemon.debug pppd[25557]: rcvd [proto=0x8f] 63 3c 90 eb 63 4b e9 ba c9 d5 46 e5 01 5a 23 a6 32 69 ad 99 a6 f5 e0 59 e7 5a 86 01 88 fd f6 8f ...
Fri Nov 17 06:29:13 2017 daemon.warn pppd[25557]: Unsupported protocol 0x8f received
Fri Nov 17 06:29:13 2017 daemon.debug pppd[25557]: sent [LCP ProtRej id=0x9 00 8f 63 3c 90 eb 63 4b e9 ba c9 d5 46 e5 01 5a 23 a6 32 69 ad 99 a6 f5 e0 59 e7 5a 86 01 88 fd ...]
Fri Nov 17 06:29:13 2017 daemon.debug pppd[25557]: rcvd [proto=0x1655] f6 c6 56 ab 16 66 2b 45 d7 c5 c0 7d 3a be 86 46 52 e8 79 09 5b fd 49 3b 9f b7 c2 e5 3e c2 9a 69 ...
Fri Nov 17 06:29:13 2017 daemon.warn pppd[25557]: Unsupported protocol 0x1655 received
Fri Nov 17 06:29:13 2017 daemon.debug pppd[25557]: sent [LCP ProtRej id=0xa 16 55 f6 c6 56 ab 16 66 2b 45 d7 c5 c0 7d 3a be 86 46 52 e8 79 09 5b fd 49 3b 9f b7 c2 e5 3e c2 ...]
Fri Nov 17 06:29:13 2017 daemon.debug pppd[25557]: rcvd [proto=0xaac3] 1c 5a 97 0c 4c a5 fc b0 83 b8 19 2a a0 dc 90 15 74 1d 25 1b 72 64 9b 6f 70 cc 7e 92 99 3b cb d3 ...
Fri Nov 17 06:29:13 2017 daemon.warn pppd[25557]: Unsupported protocol 0xaac3 received
Fri Nov 17 06:29:13 2017 daemon.debug pppd[25557]: sent [LCP ProtRej id=0xb aa c3 1c 5a 97 0c 4c a5 fc b0 83 b8 19 2a a0 dc 90 15 74 1d 25 1b 72 64 9b 6f 70 cc 7e 92 99 3b ...]
Fri Nov 17 06:29:13 2017 daemon.debug pppd[25557]: rcvd [proto=0xdf] 96 dc a7 d6 7e 83 cb d9 b7 38 52 ad 5b f5 79 15 d5 71 76 c0 70 3a 14 c2 de 0a 48 12 21 19 13 d4 ...
Fri Nov 17 06:29:13 2017 daemon.warn pppd[25557]: Unsupported protocol 0xdf received
Fri Nov 17 06:29:13 2017 daemon.debug pppd[25557]: sent [LCP ProtRej id=0xc 00 df 96 dc a7 d6 7e 83 cb d9 b7 38 52 ad 5b f5 79 15 d5 71 76 c0 70 3a 14 c2 de 0a 48 12 21 19 ...]
Fri Nov 17 06:29:13 2017 daemon.debug pppd[25557]: rcvd [proto=0xceb4] 9a d4 95 b8 ad 02 88 6a 41 af bb 23 0a b0 06 1c e5 e0 60 55 ee bd 74 79 f4 c9 50 a1 8a ed d1 fd ...
Fri Nov 17 06:29:13 2017 daemon.warn pppd[25557]: Unsupported protocol 0xceb4 received
Fri Nov 17 06:29:13 2017 daemon.debug pppd[25557]: rcvd [proto=0xfe13] 73 d4 7f a9 b6 38 c0 fc 84 b7 d9 d0 63 ce be 56 9b 11 e3 58 c1 e6 3f ad 14 6a 8d e1 88 16 6e 30 ...
Fri Nov 17 06:29:13 2017 daemon.warn pppd[25557]: Unsupported protocol 0xfe13 received
Fri Nov 17 06:29:13 2017 daemon.debug pppd[25557]: sent [LCP ProtRej id=0x17 fe 13 73 d4 7f a9 b6 38 c0 fc 84 b7 d9 d0 63 ce be 56 9b 11 e3 58 c1 e6 3f ad 14 6a 8d e1 88 16 ...]
Fri Nov 17 06:29:13 2017 daemon.debug pppd[25557]: rcvd [proto=0xe816] 0a ca 25 2a fb 17 98 82 84 f0 89 6e ba 55 34 07 80 15 6a 3d 23 11 dd e7 e2 24 7e 51 1e 0b 5b 2b ...
Fri Nov 17 06:29:13 2017 daemon.warn pppd[25557]: Unsupported protocol 0xe816 received
Fri Nov 17 06:29:13 2017 daemon.debug pppd[25557]: sent [LCP ProtRej id=0x18 e8 16 0a ca 25 2a fb 17 98 82 84 f0 89 6e ba 55 34 07 80 15 6a 3d 23 11 dd e7 e2 24 7e 51 1e 0b ...]
Fri Nov 17 06:29:13 2017 daemon.debug pppd[25557]: rcvd [proto=0xd7] 14 94 7b 52 0d 41 23 80 d7 b9 f4 86 2a cd 3c fa 80 77 15 88 fe 8c a5 51 3d fd ca a0 77 c1 ef ac ...
Fri Nov 17 06:29:13 2017 daemon.warn pppd[25557]: Unsupported protocol 0xd7 received
Fri Nov 17 06:29:13 2017 daemon.debug pppd[25557]: sent [LCP ProtRej id=0x19 00 d7 14 94 7b 52 0d 41 23 80 d7 b9 f4 86 2a cd 3c fa 80 77 15 88 fe 8c a5 51 3d fd ca a0 77 c1 ...]
Fri Nov 17 06:29:13 2017 daemon.debug pppd[25557]: rcvd [proto=0x5d] 34 8b 6c d0 79 33 ba 49 af c9 dd 41 57 89 05 2f b3 64 66 71 a2 93 98 be 80 d2 58 64 ca a7 d0 f0 ...
Fri Nov 17 06:29:13 2017 daemon.warn pppd[25557]: Unsupported protocol 0x5d received
Fri Nov 17 06:29:13 2017 daemon.debug pppd[25557]: sent [LCP ProtRej id=0x1a 00 5d 34 8b 6c d0 79 33 ba 49 af c9 dd 41 57 89 05 2f b3 64 66 71 a2 93 98 be 80 d2 58 64 ca a7 ...]
Fri Nov 17 06:29:13 2017 daemon.debug pppd[25557]: rcvd [proto=0xa1] 35 83 8a af 16 ba 98 bb b7 c1 d4 23 8e 32 60 4c bd 6e 3c 5d 0f 47 3e 5b dd 20 ca 7c 36 ee 29 ca ...
Fri Nov 17 06:29:13 2017 daemon.warn pppd[25557]: Unsupported protocol 0xa1 received
Fri Nov 17 06:29:13 2017 daemon.debug pppd[25557]: sent [LCP ProtRej id=0x1b 00 a1 35 83 8a af 16 ba 98 bb b7 c1 d4 23 8e 32 60 4c bd 6e 3c 5d 0f 47 3e 5b dd 20 ca 7c 36 ee ...]
Fri Nov 17 06:29:13 2017 daemon.debug pppd[25557]: rcvd [CCP TermReq id=0x2"Encryption got out of sync"]
[b]Fri Nov 17 06:29:13 2017 daemon.info pppd[25557]: CCP terminated by peer (Encryption got out of sync)
Fri Nov 17 06:29:13 2017 daemon.err pppd[25557]: MPPE disabled[/b]
Fri Nov 17 06:29:13 2017 daemon.info pppd[25557]: Connect time 0.2 minutes.
Fri Nov 17 06:29:13 2017 daemon.info pppd[25557]: Sent 26602 bytes, received 1084491 bytes.
[b]Fri Nov 17 06:29:13 2017 daemon.notice netifd: Network device 'l2tp-vpn' link is down[/b]

And then L2TP reconnects. These constant reconnections make Internet connection very unstable.

PPP options are default, and L2TP options are created by /lib/netifd/proto/l2tp.sh., where I added a few lines to default options:

mppe required,no56,no40,stateless                                                     
novj                                                                                              
novjccomp                                                                                                         
nopcomp                                                                  
noaccomp

So the main problem is that PPP receives a lot of unsupported protocol requests from server, and then CCP compression 'gets out of sync', and after that MPPE gets disabed and the connection is terminated.
On the other hand, PPTP works absolutely fine using MPPE encryption and CCP compression.
When I disable MPPE and CCP with 'noccp' option, L2TP works fine. but without encryption it is unsafe.
So can anyone help me fixing this trouble? Any help is appreciated.