I feel tha same: strange things happens. ;-)
If I disable "mailuser login" the message "mailuser login disabled" appears instead the other message. So I'm sure to have it correctly enabled.
I've read in the manual that it is done via pop3, so I enabled it in dovecot. Before I got the message "cannot connect to host 'localhost'". So I'm also sure to enabled it. :-)
But there is no message in the mail.log file. Do I have to setup some special thins for pop3 in dovecot.conf?