Ttyd not working

I usually use ttyd to enter linux commands on a terminal window inside luci without the need to open an ssh connection to the router.
But sometimes it happens that ttyd does not respond (that is, its windows remains black with the prompt not appearing).
When I connect via ssh and run the command ttyd start, I get the following message that I'm not able to understand. Can you tell me how to correct the error?
Thank you.

[2018/12/02 16:42:58:4862] NOTICE: Initial logging level 7
[2018/12/02 16:42:58:4872] NOTICE: Libwebsockets version: 2.0.3
[2018/12/02 16:42:58:4881] NOTICE: IPV6 compiled in and enabled
[2018/12/02 16:42:58:4889] NOTICE: libev support not compiled in
[2018/12/02 16:42:58:4899] NOTICE: libuv support not compiled in
[2018/12/02 16:42:58:4920] NOTICE:  Threads: 1 each 1024 fds
[2018/12/02 16:42:58:4930] NOTICE:  mem: platform fd map:  4096 bytes
[2018/12/02 16:42:58:4939] NOTICE:  Compiled with OpenSSL support
[2018/12/02 16:42:58:4949] NOTICE:  SSL disabled: no LWS_SERVER_OPTION_DO_SSL_GLOBAL_INIT
[2018/12/02 16:42:58:4959] NOTICE: Creating Vhost 'default' port 7681, 2 protocols
[2018/12/02 16:42:58:4969] ERR: ERROR on binding fd 6 to port 7681 (-1 125)
[2018/12/02 16:42:58:4978] ERR: Failed to create default vhost
[2018/12/02 16:42:58:4988] ERR: libwebsockets init failed