I have a similar problem. If I try to restart it via DA (I try not to now), it won't come back up again, but if I use ssh to start it, it comes up. This is on Debian Etch 4 btw
DirectAdmin is in the midst of switching from vm-POP3d to Dovecot for both IMAP and POP3. If you can search these forums you'l find complete information. If you search the DirectAdmin knowledgebase you'll find instructions.