I'm running MH 2.105 on Windows Vista.  The CM11 is plugged into COM7 and PORTMON is showing activity on the port whenever I click on the TK screen to light a particular lamp.  PORTMON shows 8 log entries per 'click' indicating IRP_MJ_WRITEs of "Length:2  .b".
After the 8th log entry, MH will announce that the lamp was lit but the lamp fails to light.  No X10 modules are operating when operating the HR12A remotes, either.
I have replaced the CM11 with a new one and the behavior is the same.  Changing the location of the RR501 Radio Controller has not changed the effect.
I have an MS13A Motion Detector and activity there does click the relay in the RR501.  Any ideas, anyone?