Backend text element - TypeError: p is undefined'

Hi,

after upgrading to 2.11.8 after creating or editing a text element and clicking 'Save' I get an alert that says 'Are you sure want to navigate away from this page?'. Clicking 'Yes' lets you continue but the text is not saved, clicking 'cancel' just closes the alert and leaves you on the edit screen.

In the console I'm getting an error from tiny_mce_gzip.js: TypeError: p is undefined

I've had this happen with multiple sites, on various versions of Contao and have never been able to find a solution.

I've tried this in safe mode (all but core modules disabled) and the same thing happens.

Has anyone else come across this or found a fix?

Thanks

Solved:

Turns out we had a custom TinyMCE configuration that was causing the problem.

Many thanks for providing your solution. All hints on the German Wiki page pointed to .htaccess / mod_rewrite related problems which wasn't my problem.
Disabling my custom TinyMCE config solved the issue. Now I have to port my custom changes to the new config version.