Blank pages after making any changes


I saw on your WPMU dashboard that you can help me with support on any WP issues... I'll try not to abuse it, but I have a serious problem that I hope you can help me with...
After upgrading to WP4.0 I cannot make any changes to the menu anymore. If I make a change and hit the save button, I get a blank page and no changes have been changed.

The same thing happens also when I try to make changes to an existing media file.

Hopefully, you can help me solve this. Just in case, I turned off the cache, but this did not help.

Thanks in advance (Oh, and I've already granted you support access)