The registration restriction was enacted to test for a few possible security vulnerabilities.
I have no reason at the moment to believe any accounts, passwords, or data was compromised, and will be lifting the restriction shortly.
The 2.1 release of our forum software has hit release candidate 2 and I'm considering their timeframe for release of the final version and the stability of the release candidate to potentially upgrade. 2.0.x is several years old at this point, although security patches have continued to be put out. If my involvement with the site IS being being stepped up (moved to address the security certificate issue today but hit a paywall issue I don't have the credentials to move past), may also look at other forum software options.