Re: Sims 4 doesn't launch - Says game is already open

@LuxKatja  If you're using a third-party antivirus, please set exceptions for TS4_x64.exe and EADesktop.exe.  If you only use Windows Defender, and Controlled Folder Access is enabled, either disable it or set those same exceptions:

https://crinrict.com/blog/2019/03/windows-10-defender-controlled-folder-access.html

If this doesn't help, and OneDrive is active, right-click its icon in the lower-right corner of the screen and select Pause syncing > 2 hours (or whatever you want), then try again to play.

If that doesn't help either, please run a dxdiag and attach it to a post.

https://help.ea.com/en-us/help/pc/how-to-gather-dxdiag-information/

And let me know whether you've moved the game's program files or the user folder to an external drive.