I may have found a workaround.
I say this rather cautiously but it did work for me.
Ubuntu 24.04, Firefox, wired connection.
Step 1:
Open a terminal and find your connection name with ip link
Mine is eno1
Run the following command (note this is a temporary workaround that is lost after rebooting):
sudo ip link set dev eno1 mtu 1320
Step 2:
Clear cookies and site data in Firefox for Discourse
Step 3:
Navigate to the Ubuntu One SSO login site https://login.ubuntu.com/ and disable “Enhanced Tracking Protection” then login with your credentials.
Step 4:
Go back to Ubuntu Discourse and click Log in and you should be automatically logged in again.
Let me know if this works 
I would be curious to know if it also works on a wireless connection.