GM:S 1.4 GMDebug.exe won't open

Hi, I'm using the Steam version of GM:S1.4 (v1.4.1804) on windows 10, and the debugger won't start with the game. The runner application starts as normal, but the debugger window itself never opens.
Attempting to set gamepadcount to 12
Start debug server
Creating Debugger server port:6502
Unable to find output device "null" defaulting to "BenQ GL2250H (NVIDIA High Definition Audio) via DirectSound"
Total memory used = 879823(0x000d6ccf) bytes
...Waiting for debugger to connect...
I've tried:
-reinstalling GM:S
-restarting computer
-disabling firewall
-disconnecting ethernet
-adding an exception in Avast for GMDebug.exe
-opening a different project
-opening the same project on a different pc (which worked, debugger started as usual)
-copying the GMDebug folder from my laptop (where it works) to my pc
-deleting the GMDebug folder and having steam validate files

As best I can figure, something is stopping GMDebug.exe from opening, but it's not Avast or Windows Defender.

Has anyone else had (and fixed?) this?