I've been getting this error for months.. Seems to happen at random intervals.. I've set up a cronjob to restart cpanel (the only "solution" I've found.. only curing the symptom and not the illness) ... I've searched on here several times for an answer, and tried all the ones I've found (/dev/null permissions, upcp --force)
This is bugging the hell out of me, cause I'll wake up in the morning and see I have no emails, just to find out cpanel is "at it again", meaning i've gotta restart it manually.. Then I've gotta wait for important emails to trickle in as mail servers reattempt to deliver.
Helllllp!
(thanks!)