I'm running on macOS and I run into this problem constantly. I have 50+ GB free on my only drive, and about 800 MB of game saves.

It's getting very disruptive since the game has taught me the hard way to save every few minutes.

What's the real reason for this prompt?
How do I fix it?

Try to move 3/4 of old saves after each play, to another folder...

Thanks for the advice, I will try.

It seems like a random occurrence however; I can delete two save games and still be unable to save. I can try to overwrite another save game and still it says the same thing. It would be helpful to know what is going on so that it can at least be anticipated.

I'm getting the same error. I have 77 GB of space free on my drive, but I got an out of space for saves error. I moved the first 135 saves out of the save folder (leaving only 20 or so in there), but it doesn't help. I get the error again every time I restart The Witcher and try to make a second save (the first save after starting the program worked fine, oddly, for the two or three times I tried).

Edit: I found a (poor) solution. I restarted the game, loaded my latest save, minimized the program, and while it was running I moved all of the save files out of the save folder.

Edit #2: Removing files from the save folder is a stopgap measure at best, and doesn't seem to be very consistent. Sometimes the out of space error stays away for 100+ saves, sometimes it comes back after at little as 5. Sometimes it takes multiple tries (restarting the program, loading a game, removing the saves, and trying to save) before it'll even accept saving again.

Edit #3: Could the problem may have to do with the game incorrectly generating a save and/or a save getting corrupted (while still appearing to be loadable), maybe? I noticed the last time this error happened that the last save I made was about 3/4 the size of a normal save, and one of the autosaves had an incorrect number (there were two of them numbered 000003, whereas the four autosaves are normally 000003 through 000006).