Just following your progress until I get the chance to test it myself. Not to digress too much from the topic but …
I’ve had issues like this as well and it may be related to the “not keeping up” error. You might want to check all your cables to make sure nothing wiggled loose, maybe even disconnect and reconnect. Check the female ends of the motor cable connections to make sure the pins are making good contact. Also, one of the shield designs out there (don’t remember the name) has a pin on the underside of the board that touches the usb housing when pressed all the way down on the arduino. If yours is like that you might want to put electrical tape on the usb housing.