Not sure but when you select "save as" "ai", is there an options tab near the bottom left of the save-location screen? If so, see if you can save as an earlier ai version.
I've found a solution that temporarily worked.
Reset the settings, workspace and preferences in illustrator to the defaults. This worked for a few saves but then the Unknown Error returned. If you are still experiencing the issue I can suggest this program Illustrator Repair Toolbox