Ok I may have located the cause of my OC-r faults.
I checked again all the connections on the VFD and the Spindle.
I found out that the connections on the VFD were not solid after all....
The screws were tight but the cables were soldered on some clamps that were held by the screws in position. I looked more closely today and found out that a couple of the solders were not done properly and the cables were moving in the clamps. Both on the 3phase outputs and the single phase inputs of the VFD. So I removed all the clamps and clamped the cables straight on the screw terminals on the VFD so they are solid now. I also connected the shield of the cable on the ground on both sides (spindle and vfd).
I didn't do a better ground connection on the spindle yet because I plan to do it when I disassemble the spindle to replace the bearings. But it is solid now too. The Washer is tightly clamped between the nut and the spindle clamp.
I hope I won't see any more OC-r from now on.