As I said in the linked thread
Dev always builds for the latest available official version (currently 0.5.1), but if your device's system has a lower version the device falls into Safe Mode and should be helped out of that state by the so called Safe-Mode-Healer which doesn't seem to kick in correctly.
So when you flashed your last app code, you got a 0.5.1 app, flashed it to you not-yet updated device and fell into Safe Mode.
You could get out of this via particle update or just flash a simple 0.5.1 sketch via Build which will auto-update your Photon.
After that you can reflash your desired code via Dev again.