Switch to the 'fixmysave' branch. This branch has the last update from before October, and just booting it one time (enough to get to the main menu, then quit) will convert your save file to the new system. After that, you should be good to go to switch back to the main branch and enjoy the latest content, and will not have to do this again.

To switch branches: Right click on the game in your library, and click Properties. Select the Betas tab, and select the 'fixmysave' branch.

This bug only affects players who last played before the first major game update, wherein we switched to a new save structure and included a converter from the old system to the new system. The Spooky Season update upgraded the game to Unreal Engine 5, which has broken the save converter, and we didn't catch it as no players had this exact situation with an older save file.

Apologies for any inconvenience this causes. So far with the reports we have, there is no risk of actual save data loss - the game isn't finishing the conversion process, and thus isn't deleting the saves. The save should still be there if this issue is affecting you.