White Screen of Death on World Selection

@JellyBabiex  Please repeat the test in a clean user folder.  Move the entire Sims 4 folder out of Documents > Electronic Arts and onto your desktop, and when you launch the game, a clean folder will spawn with no content.  (Your saves and other content will be intact in the folder you moved but temporarily not read by the game.)  Don't add anything to the new folder yet; just start a new save and see whether you get the same white screen.

Please test several of the worlds you have installed, and if you get different results, test all of them and list the ones that are currently affected.  Quit to desktop and reload after checking each world.  I realize this could be a bit time-consuming, but you don't have to put any effort into the sims you make for these tests.

Ok, the clean game works thanks - now what should I do

No, this is not the solution. My save still doesn't work with 0 mods. A new empty savegame I made works - but not the saves I've poured hours into

@JellyBabiex  Try recovering a backup version of this save:

https://sims4.crinrict.com/eng/2014/09/recover-backup-save-game/