So I'm around 4 hours in the game and saving just stopped working. Started a new game hoping it would work but nothing. I tried reinstalling, running it as admin, running GOG Galaxy as admin but no effect. I can't even overwrite the existing save file.
What should I do?

If running a 32bit machine; then the following could help- but first backup save files.
WIN 32-bit systems due to low memory. To solve it, press windows key + R key to bring up the Run window. Write bcdedit /set increaseUserVa 3072? and press Enter. Reboot the PC, unSure if there should be a space between Write and bCdedit.

I have a 64bit Win10

Please check the properties of the file where your saves are located. Look if the file is 'Read Only'. If it is so, disable that option. Now try to load the save. If that doesn't work delete the file alltogether, after backing it up in a different directory, and start a new game. If the save function is enabled copy paste your save to the new, automaticaly created, file and try loading it.