If your table passes OHM’s test, your computer or monitor will probably connect the Langmuir board to the ground. The control box may also be involved. If your table does not pass, it could be the USB shield touching the case, the isolation bushings contacting the case, etc. By no means should the control box be electrically connected to anything other than the required cables. I am willing to bet that your computer is connecting the control box to the electrical service ground in your shop via its USB port. As stated above, a 2-prong non-ground adapter will most likely solve your issue.
https://a.co/d/4V7HaA2
Eliminating the ground on your computer is not optimal, as the Tin Man’s video says. If the two-prong plug does, in fact, fix the ground loop issue, a USB ground loop isolator will be the correct fix. They are on Amazon. Langmuir can send you one, or you could get fancy and buy the “BulletProof” USB isolator from Ebay.CNC USB Isolator 1Kv -(Bullet Proof USB) UC-100 USB,USB Ground Loop Eliminator | eBay