To verify if it’s indeed the issue with diode D2, you would have to desolder it from the board and test its forward voltage drop. The diode is very robust so it’s quite rare for it to fail. I suspect the issue might be somewhere else, like a shorting somewhere else that’s causing an over-current situation with D2 and hence it becomes hot. I recommend you to measure VIN (i.e. +5v) to GND resistance. It should be a minimum of 4 to 5K ohm. Otherwise something on the +5V line is shorted.