There's a number of solutions posted in the github thread. I use an app generated password via Google for the app, and I use Tasker to complete the auto backup.

#974

When you say "verified my IMAP settings", make sure you have not used the password you type to log into your account.

Make sure you have IMAP enabled on your Gmail account (idem) and that you've used an application password.

If that doesn't fix it, please create a log of the connection attempt:

• In the app, go to
  • advanced settings
    • Sync log [enabled]
    • Extra debug information [enabled]
  • (back)
  • BACKUP (the red button near the top)
  • in the ⋮ menu
    • View log
• Take screenshot
• Use the Add files button under where you type your reply here to upload that screenshot