Flashed working code with minor changes. At some point I received the error:
attempting to flash firmware to your device gm3
Flash device OK: Flash requires modules not present on device
Changed back to previously working code, but still got the same error message.
Put the Photon in Safe Mode, but there was no change. Any ideas of what this could be?
Changed the Photon to a new one. Apparently the Photon stops, blinking green, when flashing DHT22 (temp and humidity sensor) code, using A5 port. Is one of the D-ports required?