Function with guard off seems ok. With guard on, current sourced is still ok but readings seem nonsensical. Thoughts?
It's clear to me that there's a problem with the unit. Could be something trivial like a bad switch contact, but it's up to you if you want to continue troubleshooting or return it.
Measuring voltage, for example the built-in voltage source, with and without guard might provide a bit more insights.
My first suspect would be the guard switch, since everything is fine without guarding. With the 617 turned off, can you measure between the following points with the guard switch in both positions:
1. Inner shield to COM (for me: guard off: 120, guard on: 149k)
2. Inner shield to preamp out (for me: guard off: 5k, guard on: 154k)
And then measure voltage between the same points with the power on, voltage, zero check off and again the guard switch both positions. Pay attention if the voltage is jumping around a few mV or a solid 0.000 V. For me, with guard off I get a solid 0.000 V between COM and inner shield, and some mV fluctuations between preamp out and inner shield. With guard on, I get mV fluctuations between COM and inner shield, and a solid 0.000 V between preamp out and inner shield.