When I try to open the game, it shows the initial loading screen for a few seconds but then just immediately closes itself. I've tried reinstalling the game, updating my drivers, running as administrator, and it just produces the same result every time.

Did you get an error when trying to start as administrator from root folder via payday exe?

Trying to start as administrator directly from the folder has the same result as trying to run it through Steam: The initial loading screen appears, then it closes after a few seconds.
No error message appeared when trying to do it that way.

Do you run community mods? If so - double check that you update them/removed from the game folder.

I have removed all of the mod files and verified my files afterward, and it is still doing the same thing. No visible change when trying to start the game.

Did you make sure to remove/update superBLT? (WSOCK32 or IPHPLAPI in your game folder)

Ok removing superBLT let it run normally, and reinstalling superBLT fresh is also running fine.
Haven't had to do that before, thanks for pointing that one out.