Assets in pack could not be deleted

Assets (tested with a stamp) in the Art Manager could not be deleted when added to a pack. I was just playing around with the Art Manager options a little bit and created a test pack and added a test stamp to that pack. Up until here all good. After that, I wanted to delete the pack and got the message that there has to be no more active Assets in it. So I tried to delete the asset from the pack first which does not get deleted with message popup "Unable to delete asset".

As a follow up problem I'm not able to remove the pack, too, since I'm not able to delete the asset in it.