[Dovecot] "not listening" errors fdpass.c

I'm getting a flood of errors that look like:

Sep 3 13:27:02 localhost dovecot: login: received another "not listening" notification (if you can't login at all, see src/lib/fdpass.c)

every few minutes.

Versions:

Dovecot: 0.99.10.9 Exim: 4.34 Debian Linux 2.6.8-1-686-smp Squirrelmail: 1.4.3a

Although the majority of users (99%) have no problems logging in and reading their mail, occasionally one user gets timeouts when trying to access their mailbox, seems to correlate with the errors. I can't locate fdpass.c, so what does this error indicate?