Google decided K-9 Mail needs to be verified again in order to use OAuth. We’ve started the process, but it might take a while to complete. See Unable to login to a Google account · Issue #8598 · thunderbird/thunderbird-android · GitHub
It looks like some authentication attempts are blocked because of this, but not all of them. It’s unclear to me which accounts are affected and which aren’t. Thunderbird for Android still seems to work with Google accounts.