Modpack version
1.56

Can the issue be reproduced?
Yes

Logs
https://pastebin.com/tn6XHib0

Issue
Some players have kicked a member from their Better Questing party, got booted from the server and are unable to rejoin, even after client restart.
I included the section of the lastest.log that refers to the connection attempt.

Restoring data from backup to the ~/world/betterquesting folder has solved the issue.

Cool, it sounds like a fairly nasty bug though.