Media upload breaks after pro sites activation

I have a fresh install of WP multisite. After installing pro sites I can no longer add any images to posts / pages. There are no other plugins active. Everything is more or less default. After uploading I get an error message. "Media Library" tab is totally blank.

I made a fresh install due to the same problem on previous development/testing installation. There, I noticed that this problem occured with both Pro Sites and New Blog Templates. I haven't tested NBT on this install though.