Horst,
I used the right-mouse click on the patch in the Solution Explorer, and clicked "Delete Solution". However, an error message did come saying "Solution XYZ not deleted".
The patch is no longer in the Solution Explorer BUT the Implementation Manager on the original solution does show the patch. There is a Delete button next to the patch (perhaps I should have used that first).
Clicking that Delete button in the Implementation Manager also gives the same error.
I just filed an incident. More to come.