For direct access use https://forums.oldunreal.com
It's been quite a while since oldunreal had an overhaul, but we are moving to another server which require some updates and changes. The biggest change is the migration of our old reliable YaBB forum to phpBB. This system expects you to login with your username and old password known from YaBB.
If you experience any problems there is also the usual "password forgotten" function. Don't forget to clear your browser cache!
If you have any further concerns feel free to contact me: Smirftsch@oldunreal.com
it may be a SDK but for sure it doesn't contain sources, on a first glimpse it doesn't even contain headers. So you maybe could use it to make an U3-Engine based game with Unreal1 content, but not more than that...
Sometimes you have to lose a fight to win the war.