We've had a few reports of this. It's Google's servers failing to connect, there isn't any action we can take directly, unfortunately. Google have deprecated their realtime API, https://developers.google.com/realtime/deprecation, but maybe they are slipping on maintenance prior to the closure date. We have a replacement system in the work, but it's not quite ready yet. In the meantime, switching autosave off seems to help (it's not really autosave, it's a backup, your changes will still be saved).

UPDATE November 8, 2018:

At 9am UTC November 12th 2018 all draw.io files saved in Google Drive will switch away from using Google's RT system. 

UPDATE May 3, 2018:

We've filed a ticket with the Google realtime team at https://issuetracker.google.com/issues/78443417. Please DO NOT write "+1", "Same here" etc, however, please DO star that issue by clicking on the star icon (in the top, left corner of the ticket) to indicate the importance to Google. If you are technical, please detail any traffic to the realtime servers that receives an error response. Again, please DO NOT write "me too", "same here", etc, it spams everyone who has starred the issue and dilutes the important information in the thread.