Authentication issue with embedded videos using Firefox 91

I have an issue with authentication using Firefox 91 concerning embedded videos.

I'm in a company network and have enabled Windows Single Sign-on.

The problem is, If I open an embedded video using

https://web.microsoftstream.com/embed/video/12345 (embedded, for iframes)

I get the message "To see this content, sign in. New to Microsoft Stream? sign up".

If I click on "sign in", I'm shortly redirected to the login page, the login is happening automatically, as usual, and then I'm redirected back to the video and get the same message.

If I open the exact same video using https://web.microsoftstream.com/video/12345 (non embedded) everything works fine.

I've already deleted all cookies, no success.