Casio VZ-1 Grounding issue

Hello, I have a Casio VZ-1 that I purchased back in 1989. About 15-20 years ago it developed a strange issue: If I plug it in with the standard, 3-prong power cord, when I power it on, all I see in the display is two offset dark rectangles, and I get no sound, and no response from any panel button, slider, or wheel. However, if I plug the 3-prong cord into a 2-prong adapter (essentially lifting the ground) it works perfectly normally. I went through a long troubleshooting session via email with the guy who ran the EZ-VZ website at the time, but we could not find the problem. We tried everything, different outlets, removing all connections except power, different power cords, different audio cables, etc. Any ideas on what the cause might be? Thanks! I have been using it with the ground lifted, but that doesn’t seem very safe.