The problem was solved by reinstalling the forum files.
Jun 19, 23:40 PDT
This will be a major outage from the looks of it, unfortunately, and will take more than a few hours to resolve. The trick we used to get the forums working last time they had an outage left the system in a weird, unstable state from which we ourselves cannot escape.
IPS support has been contacted, and we are currently waiting to hear back from them in the morning.
Jun 18, 01:37 PDT
During an attempt to install a plugin, the forums have decided to entirely stop working. While the plugin has been removed, the issue persists.
Jun 18, 01:04 PDT