Hi Maik,
Thanks for access, sorry 4 delay - you have problem even in Joomla Media (native core component, screenshots below).
I wasn't able to preview, upload any file nor create a new folder. This is NOT normal Joomla behavior.
Please search or ask on forum.joomla.org - how to solve, then I guess problem will be solved everywhere, in all extensions.
Something is locking that basic features, maybe .htaccess rule or hosting security settings? It's not problem caused by our products this time, I hope you understand.
On test subdomain install clean Joomla 4x (no .htaccess !) and make tests as well.