I’m not a paid user because I’m still evaluating the software.
The issue is similar to the one identified in this old thread, with a solution by developer Gary:
Unfortunately this didn’t work either.
I then removed and added the account a few times. Always there are CalDAV or CardDav logs associated with the unauthorized error dialog.
Eventually I removed and added the account and I entered the imap and smtp servers manually. This has solved the issue, but the account is mail only with no contacts or calendar.
Regards