2.8 – Meta widget doesnt show anymore

Hi, before the update to 2.8, I had the meta widget in one of my sidebars. After updating, it somehow vanished, although initially it was still listed in the active widget list in the admin pages. I removed it and then put it back, but now it simply vanishes from the list too when I reload it. Adding/removing other widgets works fine, I have this trouble only with the meta widget.

The problem is, now noone can log in to the blog anymore, nor can I directly access the admin area. Does anyone else have this problem? Or is it related to my specific theme maybe?