After playing on the server for a while, some players would get disconnected after interacting with the quest book whatsoever.

teams.json appears to cause the crash after a while; wiping its data fixes it
teams.txt
Disconnect message in server console
message.txt

Sorry but 1.12 is not supported anymore. We can barely give support for 1.16 and so there aren't any time resources to dig through 1.12 too