I haven't had this myself so I can't promise I have a solution but if it is happening with old saves, saves that have loaded successfully in the past, I would probably pin it on how the game is installed.

Have you tried using Steam to verify the integrity of your game files? You can find this function by right clicking the game in your steam library, clicking properties, and then opening the 'local files' tab. With this you can scan your installation folder and Steam will automatically redownload any files that seem to be incorrect or missing.

Also when was the last time your saves loaded properly, and were you using any mods at the time?

Certain mods add persistent features which require foreign information to be injected into your saves in order to work properly. The outcome can be unpredictable (usually bad) when a new patch attempts to read that information, especially if the purpose of said information was to influence how vanilla data was used any way. Simply removing the mod is unlikely to work in this situation because the save itself has been modded.

I hope this is not the case. The devs have done their best to make old worlds forward-compatible but they don't go out of their way to also account for every potential mod conflict because of how impossibly large of a task that would be; a task that may also inhibit how official content can be expanded or enhanced.