it has happenes twice now already, i can load old saves and even start a new game but my current save will often bug and close the game if i try to load it.

Im using Windows 10

Are mods involved?

no, i just bought the game

GOG vanilla install has Wesp's update in it. I think there's a way to get to an unmoded version, but I don't recall the method. Look through this forum to find it's location.

The vanilla version is still available in GOG. But you always need to start the game from the same shortcut! You mustn't swap between vanilla and patched version...

So, true vanilla like the game was released all those years ago and the version GOG sells are not save game compatible!

As Wesp just said, you get both versions from GOG. You just have to point to the one you want. The unmodded one is full of bugs though.

Also if you're using Galaxy to launch the game, disable the Automatic Updates. If GOG updates with a new version of the patch, and it gets auto-updated on your system, it'll cause issues with your save games.