It appears that the web hosting for this site is behind a proxy server or possibly even a dynamic dns setup and that the success or failure of the login depends on if the ip address that the forum software sees from that proxy has changed since the last login.
Under my account’s preferences tab, my location is listed as Newark, New Jersey, United States, I’m actually in Colorado, and shows an ip address, on hovering over the location display, having nothing to do with my actual public ip address. A different site using this same forum software correctly shows my location and actual public ip address.
When the login issues showed up, I (successfully) logged in with a different browser on the same computer. The preferences listing for that device showed the same New Jersey location, with a completely different ip address (the first octet between the two were different.) I just logged out on that 2nd browser, and now am getting the same error message in that browser that the other users are getting. The two instances of these logins using different browsers occurred with a large difference in time between them, and were probably successful since the ip address the forum software sees was different. Currently trying to login with the 2nd browser probably appears to be at the same ip address as my primary browser, being used to write this post.
Whatever the forum software is doing with the login, it is actually using the ip address for functionality, rather than just for tracking purposes. I suspect that only one login per username, per ip address, is allowed, and you can only login again if the ip address changes, which isn’t even in the control of the users due to how the site is being hosted.
I’m wondering how much traffic the site has lost due to this, where visitors simply didn’t attempt again once they couldn’t login?