Hello Sim4Lorn,
Don't overwrite the current game by clicking on to Backup, but click on Save as, in order to save the current game as a new saved game under a different name from the one it had when it was to its launch.
If you use the "Save" option, the save file for the current game will be overwritten. This is why you can no longer see the old save file. This file has been overwritten by the "Save" option you have just run on your current game.
It's also preferable not to launch your current game by clicking on Resume, but rather to click on Load game, in order to open the window showing all your game parts saves, and then choose the very last of them, or the one of your choice, depending on what you wanted to play at the time (an old game save, a challenge in progress and saved on another game save, or a game from one of the scenarios offered in the game and that is still unfinished).
As for the current game that could not be saved, and that displayed an error message code 0".
Check your console storage space and your online storage space. Low storage space can potentially disrupt the save process of your current game. Low storage space can block the process and prevent you from saving.
This error "Error while saving. Error code: 0" was reported in July 2023, a bug report had been opened on the English forum : [OPEN] [PC/MAC] Save Error 0 (Patch 1.86.166).
This report continued at the beginning of this year, and has been relaunched more vigorously for a week.
Do not hesitate to go there and click the Me too button of the first published report.
