Thank you for that information!

Restarting Joplin (and thus forcing it to reload its encryption keys) may help (if not already done):

  1. File > Quit (not just close) the instances of Joplin with decryption errors and re-launch them:
  2. Next click the "Show them now" link ("Zeige sie jetzt an").
  3. Click the "retry all" link (if visible).
  4. Close the sync status screen.
  5. Click sync.