Hi LH.
Siemens motors eh, the bane of my motor rewinding days. Most other motors have plenty of spare room in the slots where the windings go. So some more room for cooling, not Siemens, they fill the slots totally.
They also seem to have small frames for the HP ratings they squeeze in.VFDs create. There is mention of sparking and heating in the motor bearings and the need for insulated bearings in some cases. Sounds like it may relate to your compressor.
Also compressors being required to start under load can be a problem, if the drive is set at too low a speed or voltage the motor will not have the torque to overcome the load and will run hot.
I wonder, I helped a fellow sparky once who was having trouble with a motor on a VFD that was drawing way over the rated current. It was only a small motor but it was connected wrongly. The motor was only 1kw from memory and it was delta connected. When the drive was turned on it ran very hot, the current was way up and the VFD just shut down.
I suggested that we change the terminal connections to Star and that solved the problem.
Seems that someone changed the motor or changed the connections and it all went pear shaped from there.
Check the nameplate on the motor to make sure it is not supposed to be Star connected, that would explain the max 400 volts on the VFD. Long shot but worth a check.
Pete
Have a look at this article, it is short but mentions some of the problems
https://new.abb.com/drives/segments/motors-and-drives-in-potentially-explosive-atmospheres/effects-of-variable-speed-drives-to-motors