We are using the Fantasy Evolved II v2.1 modpack on Twitch. It has Waystones 4.0.49
I had a lot of waystones in my list, and immediately upon activating a new one that I found, I lost connection to the server.
Trying to log back in, I would complete the login, then immediately error out and disconnect.
See the attached log file:
latest.log

My server admin and I went through some troubleshooting. . . an hour later he came around to trying to edit the nbt data in the player.dat file. He noticed that the number of waystones was exactly 128. He then proceeded to remove all the data for the waystone mod. After that I was able to complete the login and play (I just have to rebuild a waystone list, smaller this time assuming 128+ caused the issue).

Hopefully this helps anyone else as a workaround until the issue is verified and hopefully resolved.