[2021] EU4 won't stop running

I don't see EU4 in Task manager there so I guess this is a Steam problem. Please try a
Steam Flush Config

Steam Support :: Clear download cache

support.steampowered.com  

Let's try a clean reinstall of Steam and game then:

- move any valued save games elsewhere
- "uninstall" in Steam-EU4
- exit the Steam app entirely
- manually delete both the Steam/SteamApps/common/EU4 AND Documents/Paradox Interactive/EU4 folders
- delete everything in the Steam folder EXCEPT steam.exe and the SteamApps folder
- run steam.exe to rebuild all that
- re-install game in Steam, run a Steam Verify when done.
- add EU4.exe to the exceptions list of your antivirus app; ESPECIALLY if you have Windows Defender, add it to the Ransomware "Apps Allowed Through" list.
- start the game with no mods active and test

Good luck with tha!