[Current Issue] Sims 3 "serious error" when loading save

louise199360​  The .bad files are unusable, so yes, delete them.  The Error 16 is most commonly related to OneDrive too these days, but it's technically a more generic can't write a save error.  Historically, it's been due to the save itself being larger than 4 GB or the save name being unacceptable.  Did "save as" produce a usable save, or only one ending in .bad?  If you can reload the save, you can ignore this problem until and unless it happens again.

Content installed through the launcher is bundled into large files in DCCache.  The files that end in .ebc are Store content; the .dbc files are third-party custom content.  If you transfer these over, that content should work again.  You'll also want to transfer the Library folder, which contains saved households and lots, both those you've created and those you've installed.

The ccmerged.package file in DCBackup is necessary to make premium Store content work, "premium" being a designation that basically says "this item comes with new interactions that don't otherwise exist in the game.  The basketball hoop is a premium item; a chess table is not since the game itself contains them too.  Point is, if you have any premium Store items, copy over this file too.  Everything else in DCBackup can be discarded.

Or you could reinstall everything fresh, if you prefer.  Anything you've downloaded will be in the Downloads folder, and you can move over these files to make the downloads appear in the launcher.