besides the error message - does sync work in general?
that particular error message was introduced with feat: add flag to detect authFailure on performSync stage (!128) · Merge requests · e / os / AccountManager · GitLab - the intention was to notify that the password used to sync an entity (contacts, calendar,…) is not or no longer valid for the account in question.
At best you changed the main password, or enabled two-factor-authentication (2FA / MFA) that enforces usage of app-passwords (tokens basically) → Solution: update password or token in Accountmanager.
At worst, the message “Your account null” points at the accountmanager (AM) doing a sync on a incomplete account entry that got created somehow.
You’d need to survey AM entries it syncs
- at Settings → Accounts how many are shown and if what you’d expect
- or deeper at Settings → Apps → See all Apps → upper right Show System → search for “Account” → choose “Account-Manager” → Additional settings in the app → use back arrow in upper left → are more entries shown than in the basic Settings → Accounts before?