Plesk users (version 8.x up to 10.x as far as I've heared) are getting errors from the Watchdog module. Parallels is coming out with an update for this error in the future, but they can't tell when (yet). These errors occur on daily basis and are actually pretty easy to fix. You might have two errors (after each other), but this fix will solve them both.
I've been upgrading Plesk today and yesterday. Although Plesk has a nice interface, it's not 100% clear in what happens. After some help on the FreeNode #plesk IRC channel and some cyberciti.biz pages, I've composed some details and commands to execute at the command line which can help when upgrading Plesk.