Cannot save image set

I've tried to create an image set with two items (small and large), but I cannot save. (see screenshot). Apparently I have not filled in something, but there is nothing red in the rest of the UI so i've no clue what i've done wrong.

This might be caused (I found later on) that the project gallery names should be unique? Later on I generated HST beans and got two files with the same name! However when I go back to the Gallery plugin then there are no image sets. (i'm pretty confused by now)

Note that the cross on the right (delete icon) inside the warning does not do anything.