The unbearable solution was to go over all galleries individually and reset all pictures.
That solved my problem as a “work-around”, but it is not a good solution at all. So I’ll leave this topic open, because it is likely that people will try to add watermarks and change them, and there need to be an easier, faster solution to it.
Maybe the “reset all” button was actually doing its stuff, but I was impatient to see a grayed screen with a circle of static bubbles that didn’t turn, looking suspiciously frozen.
Maybe it is just a matter of user feedback. Change it to a simple 0% to 100% progress bar.
Or maybe it was really frozen.
Thanks.