POP3/IMAP Error

[joximu]

Most important is: get rid of the fatal error message in mail.log

Becaus if postfix cannot read the mailbox hash config file then it doesn't know the valid mail addresses.

Can you give us the output of "ls -la /etc/postfix/ispcp"

Maybe check if there is also the same file in /var/spool/postfix/etc/ - or search the system for "mailboxes" and "mailboxes.db".
On some systems postfix copies the files to his chroot... depends on the distribution...

/J