There we go. After half an hour, I managed to register and reach the "post reply" panel.

Same issues here (in case you haven't guessed from the last line).
From Germany
ISP Telekom
Firefox/Chrome (no difference)
Windows/Mac (neither)

I kinda doubt that this is a problem with Telekom. Not only is it the biggest ISP here, but this page is the ONLY page I have ever seen such a behaviour, outside of home-owned ftp servers in the early '00s, in the past 15 years of heavy internet use.