K-9 5.800 Google critical security alert

After the update to 5.800 my phone reported a K-9 gmail authentication error on July 28 and July 31, followed shortly by an email from Google with the subject “Critical security alert”

“Sign-in attempt was blocked - Someone just used your password to try to sign in to your account. Google blocked them, but you should check what happened.”

myaccount.google.com: Suspicious sign-in attempt prevented.

Google prevented someone who knows your password from signing in to your account. If this wasn’t you, you should change your password immediately.

followed by: Unknown device Germany (this could be significant because the device IS known.

FYI: This occured both times on a Samsung Phone with Android 9 while on mobile data, in the meanwhile the K-9 has been updated to 5.801

It sounds like you provided your google/gmail account password rather than an app-password. App-passwords are required for any 3rd-party apps that aren’t using OAUTH2.

See: https://support.google.com/accounts/answer/6010255 for more details.