I can send the program to the computer (using the play/arrow button) which works fine, but the program is gone upon reboot. Using the button next to it, which should download to main.py, it instead always errors out with "Download failed."

Note: The python package adafruit-ampy does allow me to send a main.py to the unit, but that's pretty awkward, having to copy the python code from the GUI to a file for every update. But sending to the unit reliably is possible; just something with web terminal is broken in v2.2.1 it seems.