In 90% of cases the OAuth token has expired. Google and Microsoft periodically request re-authorization, especially if you have not logged in for a while or have changed your password.
Fix: go to Settings → Integrations → Email, click Reconnect next to the account in question, and run through the OAuth flow again.
For IMAP accounts: check whether the app password is still valid. Apple and Yahoo sometimes expire them after a few months of inactivity. Generate a new app password in your provider account if needed and paste it into Senly.