Resolution
Please, use the following command line to switch this option forcedly.# mysql -uadmin -p`cat /etc/psa/.psa.shadow` psa -e"update misc set val='enabled' where param='allow_short_pop3_names';"Warning: if you change this option forcedly, all users with the same names/passwords will login to one mailbox (the first one is listed in /var/qmail/users/poppasswd).
Also, you can use the attached script to find all mailboxes with duplicated names/passwords.
Keywords: long short mailnames