Thank you for that information!
Restarting Joplin (and thus forcing it to reload its encryption keys) may help (if not already done):
- File > Quit (not just close) the instances of Joplin with decryption errors and re-launch them:
- Next click the "Show them now" link ("Zeige sie jetzt an").
- Click the "retry all" link (if visible).
- Close the sync status screen.
- Click sync.