Widget page gets blank after dragging a widget

It happens so fast that I can’t even add a title and hit “Save”.

Same when clicking on a widget, choosing a location, and clicking on “Add” – I suddenly get a blank page that loads with no end.

No issues when modifying existing widgets though.

Is there a way to fix it? I’m using WordPress 3.8.1, Directory Theme 6.3 with a child theme (Practis).