I'm having the same issue, I love the game but the inability to save makes it nearly unplayable. For mine it gave the system memory in use as 255MB both before and after unloading, so it's possible that there might be an issue with a fixed memory limit (I have plenty more system memory available, but it looks like the game might just not be using it). The first time it happened I had encountered a few bugs when playing, like when I got a NullReferenceException after giving two of the same limb type to the questgiver asking for four replacement limbs, so I thought maybe the save error could be a result of that. But in my second playthrough I got the same save error again, and I don't remember encountering any bugs that time. So far I've only been able to save in the very early game without any problems.