Just hunted for several hours a stupid problem in @TYPO3 that suddenly appeared this evening. I got following errors on all pages where I had a plugin:

Oops, an error occurred!

Controller or action were empty when overriding switchableControllerActions from flexForm.

It was caused by a system update yesterday with a buggy libxml-version. Setting [BE] [flexformForceCDATA]= 1 fixed the problem, but I still had to save every plugin that was affected by this error.