This issue has been fixed with Patch 1.36.104. All your old saves should be playable.
On loading a certain save, I get either error 102 (live mode) or error 122 (build mode)
The cause is unclear at this point but it seems to have something to do with broken relationships and deleting Sims.
At this point, there is none. You can try recover a new save via the recover save game function but it might break again.
The best thing to do is save your broken save game to a safe place like an external drive and not play with it until this can be fixed. Backup your entire saves folder found here: documents\electronicarts\thesims4\
Start a new save and do not re-use any Sims from your gallery unless you haven’t played with them. This is only temporary until EA/Maxis can fix this issue.