proftpd stopped working on 2.6.32-21-generic as well after a reboot.
I checked again and the /var/run/proftpd directory had gone! I manually recreated it and proftpd started up cleanly.
However, following a reboot the /var/run/proftpd directory had gone missing again and proftpd failed to start up.
So actually this seems to be a problem with the /var/run/proftpd directory vanishing. proftpd will run fine on 2.6.32-22-generic as long as that directory is manually created after each reboot.
OK - things get even stranger.
proftpd stopped working on 2.6.32-21-generic as well after a reboot.
I checked again and the /var/run/proftpd directory had gone! I manually recreated it and proftpd started up cleanly.
However, following a reboot the /var/run/proftpd directory had gone missing again and proftpd failed to start up.
So actually this seems to be a problem with the /var/run/proftpd directory vanishing. proftpd will run fine on 2.6.32-22-generic as long as that directory is manually created after each reboot.