Reboot Illustrator, open the saved file, make changes and save changes.But AI cannot save the file because An unknown error has occurred.

Modify file preferences

Check If there are any existing preferences for file optimization that interrupt the file save.

1. Open Preferences by pressing Cmd+K or Control+K.
2. Go to File Handling & Clipboard.
3. Uncheck Optimize file Open and Save Time on Slow Network (Beta)
4. Uncheck Save in the background and click OK. 

Try to save the document again and check if you still face the issue.