I found out that since the slow PID of the FSS used Cavity reflection DC level, it is important that this path remains rigid and undisturbed from any testing. In CTN:2559, when Shruti and I were realigning North PMC, we used  BNC-T (which was permanently attached to the rack) to pick-off North cavity reflection DC signal. During this putting on and off the cable, we made the BNC-T itself loose and the connection to acromag card became buggy.

I have fixed this by connecting the acromag cards directly to the cable coming from the table behind the rack. Now connections/disconnections at the front of the rack shouldn't disturb this vital connection. Still, we need to be careful about this from next time. I had no idea what went wrong and was about to start a full-scale investigation into PMC and FSS loops. Thankfully, I figured out this problem before that.