TYPO3 Backend broken

Question

Okay, I have a weird problem and hope someone might know the solution:

Since yesterday the backend of a TYPO3 site I'm working on is broken. I can log in normally, but when I then select one of the functions in the left menu (no matter which one) this will happen and nothing will work any more afterward:

Initially, there's just the second menu, the third one appeared when I tried clicking through all the functions in the hope this would change something.

Sometimes an error appears:

"Page tree error

Got an unexpected response from the server. Please check logs for details."

But of course, I can't access the logs...

Clearing the browser cache, restarting the browser in save mode without any plugins, log into another user account - nothing works, the result is always the same.

Does anyone have any idea how to get out of this again? Apparently other users still can work fine with the page, so it seems as it's not a general problem with the installation.

Answer 1

Okay, I found the culprit when I thought about why clearing the cache didn't help while the same browser on another computer worked fine: What remains, when the cache is cleared?

Cookies.

The broken session was saved in a Cookie apparently, and deleting the cookies solved the problem immediately. :)

Answer 2

The TYPO3 backend consists of a frame in the website. in your case you see the website repeated in the frame twice.

Normaly that is a sign for broken session handling as the frame tries to build the basic backend.
Sometimes the browsers mix it up (so the information about your browser is helpful), sometimes the web server become confused.

Try to clear all browser caches.
Try different browser.
Reset all TYPO3 caches and verify all session data can be stored. If needed use the InstallTool instead of TYPO3 backend.