Are you seeing a Cloudflare error message when login is not working?
The core of the problem lies within Flarum, the app we use for the forum. It is very vulnerable to spam bots and for many months the forum kept getting spammed literally every day with many posts that moderators had to delete manually.
We have tried various traditional anti-spam plugins but none of them was able to really reduce the spam to an acceptable level. So we had to resort to blocking suspicious countries at the firewall level.
Currently, IPs coming from Russia, China, India, Korea, Pakistan and Vietnam are blocked because those countries are commonly associated with massive spam. If you are connecting to a Tor node in another country then it should work. We don't have any firewall rules specifically targeting Tor users.
I just checked the firewall and we had over 350 bots blocked from those countries today, so the protection is still needed. However we will test a different approach with a managed challenge instead of a hard block, which means that legit Tor users should be able to get unblocked by completing a captcha verification. This is not guaranteed to work though, so we may go back to the old method if it fails.