After 5 hours on it, I really wasn't sure it was possible.
During those hours I tinkered with the ExportDB file. That can't be faked as far as I can tell, although it is not necessary to load the game it does seem important its contents be right. I really wanted that file in the package as before my factory reset all my save files had that file, while none do now. Before I ever uninstalled Pets to attempt to get this working for everyone, my UC was saving and loading properly. Now I experience the same issue as everyone else.
I do know this, somewhere in the Documents/EA/TS3 folder there is something that says what can be loaded. It's buried in a cache file or written in numbers in some package, but it must be there. When I go back to my files before the factory reset, the save from that backup is loadable while none of the new ones are. Revert to the factory reset and the same problem presents itself. That even applies to EA worlds. The ExportDB file not being generated seems to be the big issue here but I'd love to know where that data is hiding.
I also tinkered a little with the export file and thought it wasn't possible to fake it. Hoped it was...
You say before your factory reset everything worked. That makes sense to me, since I have never done a factory reset (only cleaned up my game by deleting some cache files and DCBackup files (except ccmerged) and all my other worlds are working fine. So logically speaking, there is something that gets deleted with a factory reset, which shouldn't be deleted...
I just checked the post about the factory reset and I see that the only thing you keep is Downloads and Saves. Personally, I would also keep ContentPatch and DCCache, although I am not sure what they do. And what about FeaturedItems, IGACache, SigsCache and Thumbnails?
Still, I have never done a factory reset and I am also not able to see the save file. Does that make sense? You'd think I would be able to, since I have all the original files and never deleted any. Or does that have to do with that from my point of view the problem comes from UC, since that comes from you guys with the factory reset? Hope you understand, it is getting rather late...
Wish I could do more to help. I am just asking questions hoping to trigger someones mind so that someone can come up with the solution