My browser is telling me that the certificate for the forum has expired! I could only get in by agreeing to access despite the risks. The expiry date reported was today, presumably in the last few minutes as I posted something earlier without trouble.
This was a technical issue beyond our control where auto-renewal of the site certificate did not succeed as it should have done without us realising that. Unfortunately this expiry happened during a period when no staff tried to access the site or it would have been resolved sooner. It was in fact resolved within minutes of it being discovered there was an issue.
We are investigating the issue and hopefully it won't happen again.