Here's a simple way to reduce these sorts of issues:
Your power supply needs a ground reference. Running it without a ground is not safe. (note to self ... ground the supply on the printer ...do as I say not as I do). A hard ground is the best thing to do. As you have found that can create issues. A compromise is to tie the supply to ground through a resistor. Something like 1K ohms will take care of random leakage issues and ESD. You won't be as safe as with a hard ground. I'd bet your strange USB issues go away. If not check the USB ground with the voltmeter. Your computer power supply may have grounding issues. You might also have a circuit ground (wall plug) problem, I've certainly seen a lot of them over the years.
Your power supply needs a ground reference. Running it without a ground is not safe. (note to self ... ground the supply on the printer ...do as I say not as I do). A hard ground is the best thing to do. As you have found that can create issues. A compromise is to tie the supply to ground through a resistor. Something like 1K ohms will take care of random leakage issues and ESD. You won't be as safe as with a hard ground. I'd bet your strange USB issues go away. If not check the USB ground with the voltmeter. Your computer power supply may have grounding issues. You might also have a circuit ground (wall plug) problem, I've certainly seen a lot of them over the years.