Here we go again...(The sims 3 store content)

@Zebrakopf444  This is no longer a common issue, meaning the problem is likely on your side.  In case it's really that simple, are you trying to install the content all at once?  This can corrupt the .ebc files in DCCache that contain the bundled items.  The game has difficulty reading these files when they're over a certain size, and if it's installing a lot of content at once, the launcher may not create a new file when the current one is close to the limit.  So make sure you're only installing a few items at a time.

Otherwise, if you use OneDrive, that's the place to start: pause syncing and see whether it helps.  You can see the option by right-clicking the cloud icon in the lower-right corner of the screen.

If that doesn't make a difference, try installing the content into a new admin Windows account, just as a test.  Make it local, as in, don't link it to your Microsoft account.  You'll be able to launch Sims 3 without reinstalling anything, but your existing content won't be available.  That's fine for the purposes of this test and can be addressed later if necessary.

If this doesn't help either, please let me know what antivirus you use.

While testing, you can install just a few items if you prefer.  The content should work equally well (or not at all) whether you've installed a handful of items or everything you own.  And if the problem is in fact how many items you're installing at once, working with only a few at a time should clear it up immediately.