Based on what you told me I believe the issue is probably a mirrormode bug where it doesnt properly hook into the game so the script cant check if you have the quest completed, restarting the clients fixes that as it reloads all the hooks.
If you see it happening again and could provide me with logs I can see if there is anything useful I can give the mirrormode dev. Hooks not loading properly is a somewhat rare mirrormode bug, but it does happen so I guess any info we can collect on it could help getting it fixed