What did you change? Such a problem's solution should not be kept secret.
I put it back to adding the combo as new instead of replacing Booster and using the keys BBFF instead. It just adds a new combo the same way MutCrateCombo does. Exactly the same.
It does make you wonder how UT2004 keeps the aforementioned replacement from happening, though... I'm not experienced in network-friendly scripting, so would there be any reason whatsoever that replication would be required?