@evelienbaumans This error occurs when some of the data for the household gets corrupted to the point that the game can't load it anymore. Sometimes the steps I suggested work, but only if there's enough residual data on the sims that they can be salvaged. This is how I lost a handful of my first saves by the way: the game loaded an empty house, I couldn't get them back, and I didn't have any viable backups. So I started using mods, and it hasn't happened again.
At this point in the process, you could try adding mods, but their presence alone might not be enough—you might have to use the mods to find your sims, if they still exist to some extent, and put them back. If you're not interested in using mods, then I don't really have another suggestion, since you've already tried everything I can think of. It's unfortunately the case that these things happen with Sims 3.
If you'd like to try mods, add NRaas ErrorTrap, Overwatch, and MasterController, found on the NRaas site. Instructions on how to install mods are here:
https://modthesims.info/wiki.php?title=Game_Help:Installing_Sims_3_Package_Files/Setup_and_Files
Switch to another household, use "save as" to rename the save, quit, delete the cache files, and reload. Let the game clock run for a few sim-minutes, click on City Hall, and select NRaas > MasterController > Town Options > Reset Everything. This may take a few minutes depending on the speed of your hardware. Let the game clock run a bit more, then click on your former sims' household and select NRaas > MC > Add Sim. X out the filter and see whether your old sims are on the list. If they are, you can select them to add them to that household. They'll have lost some data, e.g. promised wishes and opportunities, and the household will only have 20k simoleons, but hopefully that will be the extent of it.