TL;DR: I tested, uninstalled and reinstalled the whole game, and the problem was that I saved the My Documents folder to a SSD with ExFAT file system instead of NTFS.
It started shortly after I resumed playing Sims 2 after a longish absence, to test out the new Story Progression mod. (Which is otherwise pretty impressive.) I noticed I started to get error messages about certainly newly created sims, something being out of range. Once I turned off testingcheats, the messages disappeared, but the game ground to a halt. I saved, played another household, saved and returned to the dorm, all was fine.
Next I noticed that bought objects did not stay in the dorm.
Next I noticed that new buildings did not stay in the subhood.
Next I noticed that new subhoods did not stay in the neighborhood.
Finally a new main neighborhood did not stay when I exited the game and reloaded.
This happened again even with all custom contents disabled and moved to the Windows desktop.
Either some file(s) was/were corrupted, or Sims 2 had a steadily growing problem with saving to SSD. What to do?
I moved my Documents/EA Games/Sims 2 folder to my external backup disk, started the game and let it create the standard folder. I exited and started again to check that the standard neighborhoods remained. Then I copied over the terrains folder, created a new neighborhood, empty and without subhoods, and exited. Started again and the custom neighborhood was there. Added Sim State University, premade Downtown, premade Bluewater Village businesshood. Exited. Started again, everything was there. Created empty businesshood in addition to Bluewater. Exited. Started again. New custom subhood was gone. Created new empty custom downtown, tried to play it, game crashed.
Uninstalled Apartment life. (I don't have the Digital Deluxe version, I play from CD.) Started game with Freetime CD. (Remember how awesome Freetime was when we got it? Pepperidge farm remembers.) Created new empty custom shopping hood and empty downtown. Entered them and they didn't crash. Exited and restarted. Not only where the subhoods gone, but the game did the whole "creating pets", "creating weather", "creating pets" etc. Woo. Created the shopping subhood again, exited and restarted. "Creating pets" etc. By the way, all the subhoods that I have created lie as package files on the SSD, so probably not a problem with saving there.
Uninstalled Freetime, Bon Voyage; tested with Seasons. (Remembers when the Seasons expansion made the game better, not worse? Pepperidge farm remembers.) Created custom shopping hood with 1 house, restarted, game remembered. Created custom downtown with 1 cemetery lot, exited, restarted. Cemetery gone but neighborhood remained. Created cemetery again, exited, restarted. Cemetery remained. Reinstalled Bon Voyage after deleting folder entirely. All neighborhoods and houses remained. Created new shopping subhood with house, exited, restarted, new neighborhood was gone from game but not from save folder.
Uninstalled Bon Voyage, made new testing neighborhood with Bluewater + 1 custom shopping subhood with a house. Exited and restarted. Everything is fine. Somehow, installing Bon Voyage from the CD breaks the game, even if I deleted the original Bon Voayage folder first.
And now, actually stopping for the night! Really. Probably.
THE NEXT DAY:
Uninstalled all expansion pack and the base game. Deleted folders. Reinstalled to a different location. Now the crashing and disappearing subhoods appear already in Seasons! But I already checked that it was not the SSD, since the files were there. Or...? Time to do some advanced Windows wizardry.
mklink /j "C:\Users\Itlandm\My Documents\EA Games\Sims 2" "G:\Sims 2"
where G: is an external hard disk, not SSD. Now whenever Sims 2 looks in the EA Games folder for the Sims 2 savegames, Windows sends it off to the external hard disk. Because it can! In the unforgettable lyrics of "Still alive"(Portal ending song): "Aperture Science - We do what we must, because we can!"
Created neighborhood Testingford, added Bluewater Village and Testing Hills with 1 house. Exited. Reloaded. Everything was where I put it. Now for the real challenge. Can we defeat the boss monster, Bon Voyage? (Doing various household chores while expansion pack installs from CD.) And there it is: Testing Hills, even the house has been preserved. Coincidence? I think not, but who knows until we have installed Sims 2: Freetime.
OK, it is 1 in the night, Freetime is up and running, and the house in Testing Hills is still there. Rejoice with me, you my friends! Now there is only one expansion pack left.
Apartment life tested OK. Premade lots tested OK. Custom content tested OK. Game is running better than ever, but of course I still have ideas to optimize it. Not tonight though. Probably not. It's 2AM for the second time in a row with this project.