If you want to use maildir, you will have to install a maildir capable pop3 and imap server like courier or dovecot first. This will affect of course all accounts on your server.
Fatal error: Allowed memory size of 67108864 bytes exhausted (tried to allocate 6291500 bytes) in Unknown on line 0
This error means that squirrelmail tried to allocate more then 67 MB of RAM. This is very unusual, either your email message is so large or the mailbox is corrupted in a way that squirrelmail fails to decode the message.
You can try to set the memory limit to a higher value in /root/ispconfig/php/php.ini and then restart the ispconfig server.