A few things you can try right away (in order of easiest/quickest):
Log out and log back in fully (from all tabs/devices if possible) — this often forces a fresh sync.
Clear your browser cache/cookies specifically for the Buzzen site (or use incognito/private mode to test).
Hard refresh (Ctrl + F5 or Cmd + Shift + R) when entering the problematic rooms.
Try a different browser or the mobile app if available — sometimes it isolates to one client.
Wait 10–30 minutes and retry; these sync delays usually resolve themselves.
If it persists across logouts/clears/browsers for more than a day or two, it's worth posting in the Buzzen forums (like that thread you might already be seeing) or contacting their support — mention it's inconsistent per room and login appears valid elsewhere, as that helps them pinpoint if it's a wider cache/server propagation bug.
Have you noticed if it only happens in certain types of rooms (e.g., private vs public, moderated ones, etc.) or after specific actions like joining/leaving quickly? That detail could help narrow it down further.