This seems like a pin 1 problem, which unfortunately could be a case of improper design. I would be inclined to pop open the chassis and see how the grounding scheme works (or doesn't).
I don't recommend using a ground lift on a long-term basis; this is a band-aid that is also a safety hazard (notice how there is a tab on that adaptor; it needs to be connected to the earth grounding rod at the service entrance).
I would instead inform Korg technical support of the problem and have them attempt to resolve it. I might also have an electrician evaluate the quality of your garage's safety ground and the earth grounding in general.