Restoring corrupted save file

@rostsit  Please make sure Controlled Folder Access is disabled or has exceptions set for TS4_x64.exe and EADesktop.exe, as described here:

https://crinrict.com/blog/2019/03/windows-10-defender-controlled-folder-access.html

If that's not the issue, have you specifically tried using "save as" with all mods and custom content removed?  It could take saving, quitting, and restarting your computer for this to work, and I'm not sure from your description whether you did all that or just tried to save one time without mods and cc present.

If that doesn't help, try using a clean Sims 4 folder.  Move or rename the existing one in Documents > Electronic Arts, open the game to spawn a new folder, copy over the affected save (but nothing else), load it, use "save as," quit, restart your computer, and see whether you can now save normally.  If not, please test the oldest backup in this same way, just to be thorough.

If that doesn't make a difference either, then there might not be anything you can do here.  The only other suggestion I have is mass-deleting sims and lots.  The idea here isn't to permanently get rid of almost everything, it's to establish whether removing enough of it would fix the issue.  Then you could use the 50/50 method, or an approximation of it, to figure out what needs to be deleted.  The first step is flushing pretty much everything because if that doesn't work, you'll know not to waste your time testing further.

Unplayed sims are less likely to be affected than played or modified ones, although some mods do affect all sims.  So I don't know if this is something you want to spend a lot of time on, if it's worth all that effort.  Unfortunately, there's not much else you can do here.  There is no utility that fixes corrupt saves, and without an error code when loading this save, we don't even have a hint of where to start looking for problems.