Can't save AI file on my PC - "unkown error"

I had the same issue, luckily I could figure it out wich an anomaly of the software could be. Nothing but a simple thing, nothing to do with deep preferences or some. I did not touch any of that so, always try to find something on what you draw. In my case, I filled an image with colour by accident. That little thing... I believe there are many others like this. Good luck.

Try this one 
https://helpx.adobe.com/illustrator/kb/fix-unknown-error-issues.html#:~:text=Try%20these%20steps,Go%20to%20File%20Handling%20%26%20Clipboard.&text=Uncheck%20Save%20in%20the%20background%20and%20click%20OK.

Do your files have linked assets?

A similar thread came up a long time ago and removing linked assets fixed it- one of them was corrupted.

Another solution would be to reset your Illustrator preferences (hold Shift+Opt+Cmd before startup).