How can earth fault meter work through inactive IGBTs
It starts with diagnosing the situation. The dector is very sensitive so you can use the instrument for finding the problem.


Disconnect the incomming DC and inverter connections from the AGH150W and read the effects on the IRD375. If that is wrong then the problem is limited to Bender units.


2.When the first step is ok connect the DC source to the AGH150W and read again.
If OK then disconnect the DC source again and connect the inverter and read again. 


The last step is to connect both the DC source and the inverter without running and read again.


4 Then run the inverter and read again.

You should be able te find the error by cutting the system in little pieces

From the action taken by OP it has become clear that there is no insulation error but a remaining resistance (not open) between the in and output side of the inverter.

This being the case the only way to make the unit working is to lower the thresholt resistance of the unit just below the measured resistance value. This can be done between 1kOhm and 10MOhm.