I had a large military ship that I transferred to the Shipyard TG, waiting to be scrapped. While I was waiting for a slip to open to scrap it, had a maintenance failure and exploded (it was very old). Bummer.
The next ship that was built gave that error - "[whatever] built on [planet] was assigned to Unknown Fleet" - and the ship is GONE. It's nowhere - not in the ship menu, nothing.
Well, it turns out that the Shipyard TG taskgroup is gone. . . the ship that exploded was the only ship in there, so it was removed.
Well, I recreated the Shipyard TG and set it to the default taskgroup again, but the existing ships under construction now all have "No assignment" as their taskgroup, and each one gives errors and goes POOF when it's done.
I can give myself the ships back with SM, but I can't find a way to change the assigned group on stuff already in progress. It should just act like hitting the "detach" button and create it's own TG.