The game crashes as soon as I start it

@Miiginu  Please remove your entire Sims 4 game folder from Documents\Electronic Arts (you can place it on your desktop).  Launch the game and try to start a new save; this will create a clean game folder with no content.  If it works, play for a few minutes, save, quit, launch the game again, and try to load that save.

If everything still works, then the issue is something in your existing game folder.  Move your content from the old folder to the new one in batches, testing each time to make sure you can still load the new save you created.  Any batch that causes another crash needs to be split and tested until you find the problem file(s).  Start with any mods and/or custom content you'd like to re-add to your game, and leave your old saves for last.  While testing, be sure to delete localthumbcache.package each time you quit.

https://sims4.crinrict.com/eng/2014/09/user-files-explained/

If Sims 4 still crashes even after you've removed your game folder, please run a dxdiag and attach the results to a post here.

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

I removed the folder but it still crashed.
I ran a dxdiag and attached it. Hope it helps you.

@Miiginu  The Sims 4 errors in your dxdiag point to DirectX 9, and normally, the solution would be to reinstall it.  You can try, if you want—there's no harm in running the installer from Microsoft:

https://www.microsoft.com/en-US/download/details.aspx?id=8109

But given your Windows 10 build, the issue is likely a recent security update (KB4515384) to Windows Defender.  A number of people have been unable to play Sims 4 since this update, and several of them have reported that uninstalling it made the game work again.  However, security updates exist for a reason, and uninstalling one is never recommended.

If you'd like to troubleshoot in other ways, I wrote out the following steps for someone else with the same issue.  (No word yet on whether any of them helped.)  Since not all players with 1903 and the security update are affected, there must be some other element that contributes to the problem.  (I don't have 1903 yet myself, so I haven't been able to test.)  Even if none of these steps help you, please let me know, since it'll still be useful information.

Edit:  I saw this mentioned elsewhere, and it may or may not help (and perhaps only temporarily), but please disable Origin in-game both globally and for Sims 4.  To change the global setting, hover over your Origin username, select Application Settings, then More > Origin in-game.  For the TS4 setting, open your Origin library, right-click on the TS4 icon, and select Game Properties.

Start by trying a clean boot:

https://www.windowscentral.com/how-clean-boot-windows-10

If that doesn't help, set exceptions, in all antivirus/firewall programs you have installed, for both Origin.exe and TS4_64.exe.  Please also set an exception in Controlled Folder Access (a feature of Windows Defender) for both.  It should only be necessary to add TS4_64.exe, but might as well be thorough.  Here's how to change Controlled Folder Access settings:

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

Please also remove your entire Sims 4 game folder from Documents\Electronic Arts while testing.  A clean folder will spawn each time you launch the game, as long as it gets far enough.  Be sure to delete this clean folder after each test.

In addition, temporarily disable OneDrive (no need to uninstall it) while testing.  Here's how:

https://www.windowscentral.com/how-disable-or-uninstall-onedrive-windows-10

If these steps don't work, please try all of it again, this time adding another clean boot.

First of all, thank you for all of these suggestions, I really appreciate it.
I did go through every step just as you told me. I disabled OneDrive, I deleted the The Sims 4 Folder every time I tried to start it. And nothing worked.
When i disabled Origin in-game the game wouldn't even start anymore at all. normally it gets to the point where I can select my character but then it just crashed before there was any picture at all.
I tried a Clean Boot which didn't do anything, I made these Windows Defender Exceptions which didn't do anything either and I reinstalled DirectX 9 which again, changed nothing.
I tried everything at the same time, without any effect.
The only thing i didn't do was uninstalling the security update (because i don't know how) but i updated it....also no success.

Not sure what to do now. Wait for another security update?

@Miiginu  At the moment, I'm out of ideas.  Yes, Microsoft should release an update that fixes the issue, sooner or later, but I have no idea when that would be.  In the meantime, I'll ask around for other ideas, and if there's anything worth trying, I'll post back with more suggestions.

@Miiginu  Do you happen to have a tool like MSI Afterburner or RivaTuner Statistics Server running?  If so, try disabling it.  Tools like these should be disabled in a clean boot, but at least one person has reported that RTSS was still running, and disabling it fixed the crashing issue.  You should be able to see it running in the tray, and bring it up and close it from there.