I found something that worked for me.
First of all, the folder for the saves did not exist, which made me wonder if that was part of the problem. So I just created a 'Saved Games' folder where it is supposed to be. If this actually helped or not, I don't know.
Then, what I actually think helped, was that I saved on a new save, instead of overwriting the exisiting save. It worked, so I removed the old save! And so, I removed the existing Quick and Autosave (in game) and then Quicksaved and Autosaved (by entering a new area), so they wouldn't anything to overwrite aswell. It all worked too!
Then, how ever, after some minutes of playing, the game crashes anyway. Not by saving though, just by me running. I think this has to do with my hardwares experiencing too much work. It is a MacBook Air after all, and I think I can probably boil eggs on it when I play PoE. BUT, when I then relaunched the game and tried to save, the initial problem was back. So somehow the random crash caused it, I guess? I dunno, but I hope this might help in some way.