This is specifically aimed at the current Fedora deployment; I'm not aware of better place to report.

I'm trying to connect to https://chat.fedoraproject.org/ using desktop client from flatpak;
I've tried Ferdium (https://flathub.org/cs/apps/org.ferdium.Ferdium) and Cinny (https://flathub.org/cs/apps/in.cinny.Cinny); the behavior was identical:

1. Connect to chat.fedoraproject.org
2. Click on button "Login with your Fedora account".
3. On the authorization page (see image), enter credentials (verified correct by logging in with regular browser).
4. Got "Authentication failure" message.

On console, Cinny reports the following:

libsoup-WARNING **: 11:44:03.137: gssapi step failed: Unspecified GSS failure.  Minor code may provide more information: SPNEGO cannot find mechanisms to negotiate

Another thing to note is that when I managed to trick Ferdium to open https://accounts.fedoraproject.org/ directly, I could log in with no problems; seems only when I try to authorize new service (element), the flow fails.