TYPO3 Core

Hey,
a normal editor was no longer able to edit content elements text/image. All other content elements work fine.
Friday was the last login and I could see (protocol), that the beuser has edited and saved the document. But monday morning it was not possible to to so.
By the way - all other editors didn't had this kind of problem. All User rights are from groups.

While switching to the user, I could reproduce this behaviour: Klicking on the pen symbol of the content element, the user only gets the tabs (General, Image...), nothing else is shown. No error message.
An accident? So I turned debug mode off again - and the beuser can't edit the element.
Turning on the debug mode again - everything works fine again.

So what happened here?

Best,
Andrea