External monitor displays "Input not supported" when MacBook Pro lid is closed

Hello,

I have a 16" MacBook Pro and I am trying to display to an external monitor while the laptop is docked and closed. This function has been working since I bought the laptop, but has recently not worked. I have the laptop on AC power and hooked up via HMDI with a USB-C hub. I also have a wireless keyboard and mouse connected with Bluetooth. The monitor displays fine when the laptop is open, but as soon as I close it, the display turns black and displays "Input not supported." Curiously, the speakers still output sound even though they are hooked into line-out on the monitor itself.

So far I have tried messing with the resolution and color profiles, which haven't seemed to solve the problem. I also tried to clear the NVRAM.

Any help would be appreciated.