Okay, in the network-tab I see that the uploader is returning:
<b>Warning</b>: preg_replace() [function.preg-replace]: Compilation failed: unknown option bit(s) set at offset 0 in <b>/home/wp-includes/formatting.php</b> on line <b>1074</b>
I will investigate in this direction.
Thanks for your reply. I disabled all plugins, but the problem still persists. I already use the Twentyfifteen-Template. Strange thing, I assume nobody else is having this problem with 4.1?
