If you are using a version of draw.io and saving fails, for whatever reason, you can obtain the XML of the diagram and manually save it.

In the menu select Extras->Edit Diagram, select the text that starts and end with <mxGraphModel> tags, open a text editor, paste the XML in as a new file and save it. That file will be loadable back into draw.io using a fresh browser tab with www.draw.io loaded, opening the file from the local device where you saved it.