I don't remember and I'm not sure if this will work, but...
Go into your Lucasarts/republiccommand/gamedata/system folder and find the UnrealEd.int file and move it to you desktop so there isn't one in your system folder. Then restart Unreal Editor. It should rebuild this file. but I wouldn't delete it just in case.
If that doesn't work then reply to this post with the last several lines of the UnrealEd.txt or attach it to your reply.
Also have you changed anything with your computer since the last time you ran the editor?