Cant insert Header anymore!

Hey, im using the Caitlin Template and after a while i wanted to put in a new header.
so i went to the template, / styling / Header Images and deleted all old ones.
But when i want to add a new one, nothing happens!
i go to list/add image ... and the windows with "Header image +- shows up. when i go to "select" ... nothing happens!
Doesnt matter what browser i usw .. even tryed it on the handy.
am i missing something here? doing something wrong?
i run joomla 3.8 stable
greetings
yogi