I have not investigated this much, as I've only had it pop up once, and I have the Steam version, so this may not help you. If you right click on the game in Steam and open the properties to verify game cache(backing up the files is suggested), it may fix the problem.

Wrong forum, buddy. This is about Gog's version, not Steam's.

Ever since I made the purchase there have been some issues with this game. There was a huge lag at first. Then it got fixed somehow.
Now, I have some characters missing, and the game still fails to synch with the cloud, and no one is even interested in investigating the issue, let alone resolving the issue. I regret this purchase.
I should have stuck with the cracked game.

It turns out cloud synchronization corrupted some of my save files. Everyone should turn off their cloud saves.
I have managed to recover my characters.

How did you recover your characters ?

I remember it had to do something with the save file text files and folders. I have compared the text files of my valid saves with the text files of the corrupted saves.
I was able to spot the difference, and edit them, and if I recall well, create the missing folders inside the save files so they could work again.

If I manage to turn on my light bulb again, I will post more datails on this. I forgot to write down what I did, so I could remind myself.

I remember now. I had the old saves backed up. However, I was unable to simply delete the corrupt files and replace them with the old, backed-up ones. I had to check out which folders and text files were missing, as they also got corrupt when I put them in the save folder. It seems that once the cloud synchronization corrupts save files, only the back-up files will do, but with a bit of fiddling with them.