I've been trying to figure this out myself for a while, but I haven't found a solution. The last Windows update (I believe it was March 4th) there was an update and now my game won't load properly. I've tried several things including taking everything out, and playing it -- not working, only getting the black screen, then I put every single CC item and expansion in my game and then it worked, and then it stopped after loading TS3 twice -- black screen again. i've tried backing my computer up too and still won't work. I even saw some people talking about it on reddit , but seems like they don't have a solution either.

I was hoping there would be another update, and then maybe that would work, but it hasn't happened yet. I've tried to fix this myself, but I'm mentally exhausted trying everything I can. It works when it feels like it, I'm not sure what I'm doing. I feel a bit defeated because I haven't really launched TS3 this month.

If anyone has any advice or experienced the same thing, please let me know or tell me what worked for you!

Thanks

Most likely the problem comes from OneDrive, in case you have it. Make sure you disable the app's access to game files or disable it completely if it's not in use.
Luckily you can uninstall some of installed updates if you think the issue comes from there.
If you use lots of core mods, some of them are not recommended to use together, so check if you have any mod conflict.

yes, Windows Update has changed something in the way the game runs. What we're getting is a popup that stays in the background that says "unofficial game mod detected." You can ALT-TAB switch to that pop up, click CONTINUE and it will work again.

I haven’t encountered this yet - maybe because I put WU on pause and haven’t received this update yet, which causes the issue. But sooner or later I’ll get it, for sure.

https://thesims3help.tumblr.com

I think the only core mod I have it the overwatch and error trap. I'm not signed into one drive if that makes a difference? I never signed in since getting this new computer (I think I got it in February). Do you know where I would find out what it has access to?

I tried this too, and it won't allow me to click it, it's like it's there, but it won't pop up and won't give me enough time to click continue. I saw it on this post on tumblr and I thought there was hope, but yet again it didn't allow me to click it.

That's good that you haven't signed in - that makes using your computer somewhat safer according to me from Microsoft's slop.
Unfortunately I can't help you find out if OneDrive has access to game files because I don't use it. There should be tutorials in YT that are helpful in this situation.
But again, if you are not singed in there's nothing to worry about I guess.

I remember that there was a link in this post, from where you could download a fix for this issue - they are links to SFS update packages of the problematic mods.
Make sure ErrorTrap matches the type of game version you have. Personally I have the disk version of the game and my ErrorTrap is for disk vesion. Here is a link to the mod: https://www.nraas.net/community/ErrorTrap#tuning. The icons below the description are download links and they refer to the game version you have.
Also make sure you use the latest vesion of the other mods you have.

@flloetryresident
Did you manage to fix this by updating your mods?

I found the error in my game. it was this mod: https://modthesims.info/d/678484/plumbot-fixes-mod.html

Its set to the disk version of 1.67. I used S3PE to edit it to the Steam version, and no more pop-up.