From what is understandable it looks like the game is trying to modify a joint but gets an invalid name. This seems linked to some in game object with custom slots. You can look in that direction already. But yeah, you might end up having to use a 50/50 method to sort out your mods.
Did you even disable the CC content to see if it works properly without CC? Cause if not you should repair your game at the very least through the launcher.
You should try from another user account on your computer. Simply create another admin account that you will delete after the test. Login into the second user session and try to install S4S from there.
If it doesn't work, it means this is a system wide issue.
You can try to have a look at your event viewer but as nothing happens it is unlikely you will find any error in there. I don't suppose you have a second monitor plugged in the computer like a TV for example ? It can happen that a computer pops some windows on the second display even when it is turned off.
You can also update your windows version with the last updates available if this is not already the case.
If none of those work I guess you are in for a windows reinstall.
This looks like a Last Exception error You should look into your /my documents/EA/sims4/ folder and check the last exception files created in there. Look for the most recent and either post the content here so we can have a look or try to see for yourself already if you notice any name of mods that you are using in there. They would cause the issue.
When saving as DDS on a mac you need to select the format in an additional window afterwards. There you need to select: DXT5 and tick the tickbox for mipmap. Leave the rest as is.
Or you simply save as png and import that, it works as well.
About the preview, there is a preview issue on Candy Apple and CAS items I am unsure until which extend it goes but it could be it applies as well for simple texture changes. To fix this and force S4S into actualising the preview, you can save your project after import. Close it and reopen it, it should update the preview.
The devs are aware of the issue.
For the turning black when moving the issue, it's probably linked and might be fixed with the next patch. In the meantime you can simply not move the window around :p
I had a look at your DONE.blend file I have no idea how you managed to fuse a full body sim's body inside your pants object without it being selectable or appearing anywhere on the UV. But this is the issue Also I though you could simply select the vertices of the pants then separate them to another object but then the new object is empty...
I don't know how you did that to be honest.. I don't know blender enough. Although I am curious as to how so in case you know what you did..
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