If you receive the message "Sharing is unavailable at this time" persistently when trying to share a diagram with another Google account, the issue is most likely a cookie on the google.com domain has gotten itself into a state where it's not able to refresh. Try clearing cookies, or, if you know how to, clear just google.com domain cookies and try again:

Note that you can also go to https://drive.google.com, find the draw.io file, right-click it and select "share". This will bring up the same sharing dialog. The sharing dialog is provided by Google. Please see this thread.