Hi Andi,
RX diversity lost issue relate to an anomally on the receiver path.Its is similar to VSWR but for its occurrence on the RX path. Try and swap your TX leg for RX and monitor if VSWR alarm will come up. If this happens it then means there is a problem on the feeder line and a feeder/Antenna sweep should be done.
You should also do a Diversity Supervision measurement on the affected TRX.