No main program specified

You would get this error when you have work that has no (longer) a science application appointed to it. This can happen in scenarios where the project runs through alpha and beta applications and have discarded some of these, while you had a large cache of work with tasks marked to run on one or more of these applications.

As far as I can find, this error was on BOINC 5 and 6 only.
Newer BOINC have better messages (via the Notifications tab) about what happened here.