I get an error message using Firefox; "An error occurred. Please try again or contact your administrator."

Hi,

When I use the Firefox web browser I get an error message when I want to post message: It is a little red box in the top right corner with the text: "An error occurred. Please try again or contact your administrator. ". I use Ubuntu 20.04 and installed Firefox using apt system. However, if I open the website in a private window and log in, everything works! That is how I posted this message. So there must be something that the normal firefox session is blocking. The question is, what? Do you know how I can fix this?

Regards,

Niclas