Cant´t open in Firefox anymore (406)

Hello since yesterday I only get an blank page if I try to open
The Web Developer Tools say theres an 406 Error
How can I make it work again?
I am using Kubuntu 18.04 LTS (32Bit)
Firefox 86.0 (32-Bit)

Is there any add-on that might be messing with Accept headers? Based on the definition of 406 Not Acceptable something like that seems the most likely. The Developer Tools should let you see those headers, too.

I´ve tried it in --safe-mode but not works.
By the way opening the link in the eMail in firefox gives wrong mime type.

There are three: Content Security Policy: Die Einstellungen der Seite haben das Laden einer Ressource auf inline blockiert (“script-src”).