hmmm. I filling tanks with FSP advanced mode. I never tried to open emergency tanks x-feeds inflight. but i did it onground while checking fuel systems with FSP already running.  And i opened and closed X- feed inflight on every flight.

My bug experience came when i started recording in FS-Recorder. After that FSP disconected from FSX and gived me that bug.
I dont use INS, so cant tell. But trying to fly it without INS good idea.

Anyway i think its Net Framework problem. trying to reinstal MS Net 3.5 may be good idea too.