Scratchbuilt Printer: Pronterface Won't Connect to RAMPS 1.4

Pronterface refuses to connect to my printer. The monitor just says "connecting..." and stays that way. Both the board and pronterface are using COM 8. Baudrate is 250000 on both. I have tried changing the baudrate. I have tried a different USB cable and port. I have made sure that Arduino and Pronterface are not running at the same time. The board can connect the the Arduino IDE without a problem, and uploads the code just fine. I have tried uploading some test code, which works perfectly, moving all motors and heating up the hotend. I am assuming that it is a firmware problem, but I don't know where to begin. I have tried two different versions of Marlin with no success, as well as resetting the board. Nothing has shown up on the serial monitor.

Whenever the board is unplugged, I get this:

[ERROR] Can't read from printer (disconnected?) (SerialException): call to ClearCommError failed
[ERROR] Can't write to printer (disconnected?) (SerialException): WriteFile failed (WindowsError(22, 'The device does not recognize the command.'))

Using RAMPS 1.4 with Arduino Mega. This is the second board that I have tried.

Here is my config: https://pastebin.com/CX7BaBEt