Frustrating that more help isn't forthcoming, huh? I have the same error show every few days. The only scheduled task I have enabled is the PurgeUsersOnline. In that task, I've changed the frequency from every 1 min, to every 3 min, and unchecked the "retry after" setting. That was the suggestion from another post I started in another forum. Unfortunately, the error still shows every few days.
The error used to show up more often, but I started doing more of the site maintenance tasks manually, such as cleaning out the recycle bins in each portal, synchronizing the database and file system thru the file managers, reindexing thru the search admin, and clearing the logs & cache. It's a real pain, even with 4 small portals, but seems to help keep the site responsive, and fewer errors!
Still, I get that error more often than I care to, and wish someone would come up with an answer. Sorry this wasn't much help (except maybe to keep your thread alive in hopes of a fix).
Rick