I am having the same problem: snapshot V3.0 cannot be uploaded on v3.20. I remember a similar problem with CPE210 v3.0 to v3.20. Snapshot V3 worked then on the CPE210 V3.20. Perhaps the same changes can be done to the snapshot image?
[ könnte etwas mit der tplink_safeloader support list zu tun haben, mehr info in dem thread: https://forum.openwrt.org/t/build-for-cpe210-v3-20/68000]