So I have the same, Win11 pro. If you downloaded from Steam, I'm at a loss why it works for you and not for me. Last resort will be to fully uninstall and try again. Here's what I've tried since yesterday, with no luck:
 
	 
 
	1. 
	- Not a perm fix, but If you want to play without saving your profile: start MP (as admin?) and go to options > multiplayer > cd key, and enter it.  
	- select co-op/single from menu. When it loads create/load a profile. Now you can switch back to MP to play, but your data is lost on exit  
	2. 
	Step 1: Launch CoDWaW 
	Step 2, press ctrl alt and delete at the same time to open a menu 
	Step 3, Open task manager, Kill CoDWaW 
	Step 4 Launch the game, It should ask you to run in "Safe Mode" 
	Step 5, Run in safe mode 
	Step 6 Create a profile 
	3. 
	- Under players\profiles, (users, appdata, local, activision\codwaw\players\profiles) delete everything. Create folder HarryWezer and edit active.txt to replace $$$ with HarryWezer. 
	- Right click on steam icon and select run as administrator. Then go to game folder, and right click on codwaw.exe for single player and multiplayer and select run as administrator. 
	-open steam as admin. Then start game NOT from steam, but from game folder, as admin. Now your profile should load. 
	4. 
	- Port forward to 28960. The ports are 28960 on tcp and udp, and 3074 on udp only. 
	- Launch multiplyer game (NOT from Steam) 
	- Enter CD key in game. Validate it. 
	- Exit game. 
	- Start game again as per above, and create profile.