When you have an existing UTmail+ account set up and are encountering issues, you can delete the current account before adding it back to troubleshoot. The error may have arisen because the original account used legacy authentication, which requires you to delete the account and re-add the account while utilizing modern authentication.
Note: If you have a custom configuration with locally stored folders and mail, please back it up before continuing the deletion process.
Consult the following articles to re-add the UTmail+ account: