Tried to "Save As" a .ai file, from a .eps file, in Illustrator and got "An unknown error has occured" so created a new .ai file and pasted the content from the eps into that. When I try to save, I get the same "An unknown error has occured" error. No locked layers etc.
Basically can't save anything as a .ai. What could be causing this and how do I fix it? Running Illustrator 25.2 (64-bit) on Windows 10 Pro.