Instrumentation Amp design for breaking ground loops between systems
Probably there are issues with the input common mode range of the inamp. They are very peculiar in that, depending on the internal architecture (often there are 2-3 pages in the datasheet devoted to the CM range in various condition of use).
The 100 ohm is often also a safety drain for remote ground lift: same issue but different purpose.
Maybe you need an insulation amplifier if you have these problems (they are not cheap)