So, some of the bits of information that I'm thinking about:

Spindle On/Off appears to present power to the spindle motor. But also there's some current flow into the metallic frame of the machine - resulting in a light shock sensation. The VFD is indicating a fault with the motor drive output - notionally a short circuit, but it could be through a return earth.

I this point I'm reluctant to support you probing around because my thoughts are that the machine is electrically unsafe. There's an amount of testing you can do with the machine unplugged - but I need to ask if you have a DMM (multimeter) and are confident with the use of one. All testing will be done unpowered.