I have installed a new cheat protection on the SS UT99 servers. It is still beta, so i'm sure there will be some bugs here and there.
Once you join the server, you will get a screen that pops up telling you that you need to install some additional files. Down at the bottom of the screen, select "install all files". After the files are installed, select "reconnect" and you should be good to go.
Let me know if you have any problems.