I'm running a small snapshot server with a few people, and we've been experiencing people having trouble joining the server. They join for a few seconds and then disconnect. It's hard to find out what the issue exactly is, but we cannot reproduce it in 1.16.5 on the same server, with the same people.

After a reboot, the problem seems to be gone for a while, only to return afterwards within a couple of hours.

Interesting thing in the console (might be unrelated of course):

[03-Apr-2021 09:54:17] [WARN] handleDisconnection() called twice

Attached are debug logs from debug start/stop, and debug report.