Just a note for future reference.

If you log in to Usermin as root, you can't read email. Any other users have no problem.

Jamie had already troubleshooted the problem and concluded that nothing can be done about it: "Dovecot has a hard-coded restriction against logins by users with UIDs less than 99, which includes root. There is nothing that can be done about this unfortunately.. The only work-around is to create an email alias which forwards all email to root to some other user or address. This can be done in the Postfix module on the Aliases page."

Update note:

The above problem only occurs when you use IMAP at] Webmin] Usermin Congiguration Module] Read Mail.

If you use Qmail, for example, no problems.