From R. Abbott's log in LLO's alog:

Ok, this sort of thing is hard to describe, but I'll try:

1.  I assume the problem you are describing is associated with the flange end vacuum side cable interface
2.  This PEEK body of this type of connector is held together by several screws that are only visible with the connector unplugged from the flange
3.  The body screws I just mentioned thread directly into the metal backshell.  Here's the point of badness.
4.  If the heads of the screws stick out whatsoever, they will contact the inside metal surface of the vacuum flange.  This is probably where you are getting your grounding problem.

As for the OSEM ground, armed with the understanding of loose screws and grounding problems, I would look carefully for a similar mechanism.

See his full log in response to ground hunting at LLO:
https://alog.ligo-la.caltech.edu/aLOG/index.php?callRep=9810