Try connecting to the Ubuntu Discourse site using VPN. Choose another European country.
Alternatively, or perhaps in addition to the above you can give this a go:
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:
Try to connect and log in again