A G21 is a drive fault. It means the drive sent the shutdown command to the mcss. It's a generic code for a drive fault. Sorta like a MLS drive Fault on a gem controller. What type of drive fault are you getting? I believe if you're picking the the brake and the switches you are causing a brake fault(unintended movement) in the drive causing a drive shutdown.(g21). Not familiar with the high voltage interface board or the relay DOLD on it. We haven't had many drive problems with the few in my area.