It seems that the underlying Discourse software used for the forum is blocking a LOT of legitimate browsers right now and preventing people from accessing forums (among its numerous other problems). It looks like the a newer version got pushed out here and started blocking more browsers today.
This is a huge and inexcusable problem with the forum software (and affects other forums as well), but I don’t think that Discourse allows the browser detection to be turned off. So, for the people affected, you if you are running an adblocker like UBO, you can add this script to block the browser blocker (at least until Discourse developers decide that they want to block you again for no reason):
/browser-detect*.js$important,domain=forums.unrealengine.com
Anyone that has been using a custom user-agent to get around this, that no longer seems to be working.