First off I need to thank someone for giving me this information.
https://steamhost.cn/steamcommunity_com/id/sh1k1


How to fix this:
Simutaneously press
Alt and Enter

Then navigate to here in your files:
C:\Program Files (x86)\Steam\userdata\58020027\221040\remote
Or
C:\Program Files\Steam\userdata\58020027\221040\remote

Look for "savedata"
Now rename that savedata to something like: savedata1
Now go back into the game and try to save, it will say it can't find the save data and should ask ot make a new save file.
Say yes and it should work.



50k Space Required
How to fix this:
Simutaneously press
Alt and Enter

Then navigate to here in your files:
C:\Program Files (x86)\Steam\userdata\58020027\221040\remote
Or
C:\Program Files\Steam\userdata\58020027\221040\remote

Look for "savedata"
Now rename that savedata to something like: savedata1

Now, exit to the menu, and go back into the story or whatever you were doing, after this get to a checkpoint. Exit the gamemode, and then go back and see if it saved.

If I'm right, it should've made a new save.







~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

(ORIGINAL)

This seems to be a localized problem affecting only a limited number of people, myself included from time to time. What I did was that I forced the game to windowed mode (Alt+Enter), then navigated to the save data directory in my Steam installation folder, and deleted/renamed my saved data file (savedata.bin). That way, the game still kept all my stuff in RAM, and when it asked me to try saving again, it wouldn't find the save data, so it requested to create "new" save data (with all my junk up until that point). I accepted, and problem solved, my data got saved successfully.

You might want to try that out. I know it's only a workaround, but hey, as long as it works until it's officially fixed...