Panel content window stays at status 'Loading...' with subtheme

Somewhere in the last few days an issue occurred that when i try to add or edit content to my panel, the pop-up window where i should define the content stays at the 'Loading...' status. When i switch back to the basic Zen theme everything works fine.

I don't have any custom JS/jQuery code in my subtheme, so that can't conflict with anything. I've only changed some CSS and content, but when i reset all the stylesheet modifications the problem still occurs.

It worked before because i've added content to the panel in the first place.

Can this be a bug in ZEN or did i misconfigure something somewhere?