Modpack version
1.72

Can the issue be reproduced?
unknown

Logs

Issue
I connect to my friend. 1) He can create party and me too but i can't accept his invite.
2) if he accept my invite to party, quset counted to me but after
disappear immediately and again counted and i can't claim reward.

Thanks for the report. We are investigating this issue on the Expert pack's issuetracker. I highly recommend switching to a dedicated server if possible.