All that this alarm is saying is that you have a difference between you main and diversity anttena lines. I have 5 years experience with Ultrasites and Talkfamily before that. The alarm parameter is set in your BSC so make sure that the parameter is not set to low usually less than 10dB of difference is acceptable.
Next if this parameter is set correctly go to your BTS and complete a return loss system sweep and make sure that your two lines that you are recieving on are passing to your spec on the uplink path. If they and you are using a tower mounted amplifier(TMA/LNA) make sure that it has the correct amp draw. One of these should more than likely be your problem. If not you may also check that all of the cableing inside your cabinet is correct and tight.