Thanks for the additional details. You could try to save the work as described here:
https://desk.draw.io/support/solutions/articles/16000089553-how-to-save-a-draw-io-diagram-when-standard-saving-failsand test if you see the same error in other browsers and Incognito mode. Also, clear browser cache in a preferred browser.
When it comes to lost edits, I am afraid I am not able to help additionally :( I would suggest to save files to Google Drive or OneDrive because they support version history.
Since autosave was on, not really sure why the latest edits weren't saved.
Kind regards,