this was happening to me too, and the only fix was reverting to direct x 9 and reloading the game. i still get the error message but if i wait a little while without dismissing it, the detail page behind the error message disappears and the lot is in fact saved properly.
interestingly, i've also noticed my game is far less prone to frequent random crashes on dx9 than dx11, which is strange since dx11 was supposed to improve game performance 🤔🤔 my loading screens are much faster, along with saving the game; on dx11 saving would take around a at least 50-60 secs every time (save file size ~36MB) whereas now its always less than 10 seconds, and loading screens when travelling to other lots went from 5-8 minutes to 1 minute or less.
what's up with that? why did dx11 make my game run so badly? i never would've realised this if i didnt try switching back to dx9 to fix this issue specifically