Just had my save games vanish.

To fix this, run the game and create a save.
Go to the games install folder Serious Sam Siberian Mayhem/userdata/
You will see more than one profile (a string of numbers)
copy your save games from the old profile (saved games folder) to the new profile (saved games folder)

Sorry doesn't work please delete. Now says saves corrupted

If you run the game with GOG Galaxy you must then in future run GOG Galaxy first before the game launches, the saves will then be ok.

Yeah, I had this happen with the original SS 4 when Galaxy touched the game and messed up the save profiles.

It created one profile for when I installed using the offline installer. Then when I updated through Galaxy, it created another one and swapped to the new profile. Original saves not visible in-game, trying to place them to the new profile made it so they couldn't be loaded (the corrupted message).

Basically, if you installed through offline installer, make sure Galaxy never touches the game.

This is wrong, I've installed game on Galaxy and it broke saves, just as in original SS4. It has created profile user 0 when first launching game, and after that, it created some other profile. And of course save games wont be recognized after being copied. What is the point of securing gamesaves in such stupid way in single player game???