I found a thread about a similar problem, but it’s several years old, so I thought I should start a new one.
Recently I discovered that the Mail application was unable to obtain messages for my Gmail account. I couldn’t find a way to update the login information for the account, so I removed it from Mail and then added it again. This resulted in the Account Manager having me log in to my Google account. Then Mail was able to retrieve mail messages.
However, I then began to receive notifications from Davx5 about Holidays in United States (Google Calendar import), even after I turned off synchronization for that in the Calendar application, and for Contacts. Both notifications say “Authentication failed (check login credentials)”. These arrive every few minutes.
I redid Oauth login for the Google account under Settings → Accounts, but that didn’t help. Trying to do it for my @e.email account gives the message “You can configure only one Murena.io account on your device”. I then deleted the Google account under Settings → Accounts, but continue to get the notifications.
As a work around, I have deactivated the US holidays calendar and set the synchronization intervals to once per day, which seems to help, but I would like to get back to how things were before.