With the motor mounted to the frame and the ground wire connected to the motor, use a 1000V megger and test the insulation from where they are removed from the VFD, all the way through the motor. I normally clamp all three motor leads in the ground clamp at once and then touch the probe on ground before activating the megger. If there's a disconnect/terminal box/other terminations involved, you will be proving them all at one time, preferably before disturbing them. I have seen disconnects with ground faults that go away when you jiggle the handle, or open the box, and then return in the middle of production.
You need at least 2 megohms for a PF4 or 40 in v/hz mode, but I have seen some drives tolerate less, while others like some Toshiba G7s I have, that want over 100megohms to avoid tripping for ground faults.
I suspect you will read much less and find a damaged wire or motor winding. If you get a low reading, leave those wires off the drive, separate them from the motor leads and repeat the test.
The wiring, open at both ends, should read infinity or as near as your meter can to that, and the motor should read above 2 megohms.
The VFD is trying to protect you from a serious shock hazard. There is dangerous potential reaching the case of the motor, I would be 80% sure it's in the motor, at least inside the peckerhead, but prove it with a megger to be sure, and test the motor leads while you have the tools out.
DON'T apply 1000v meg test to wiring that is connected to the VFD.
It is remotely possible that the tech is seeing a carrier frequency induced voltage when the ground is removed, but the fact that the drive sees a ground fault...most likely the drive is right.