Hi John thanks for your post. As a first step could you double-check whether the page you are trying to export is a Master Page? If so you will need to convert to a normal page before exporting (left click on the Master Page tab below the canvas and select 'Convert to Page). 

If this doesn't work would you mind reviewing the steps in our Export your Documents article? Let me know if you have any questions about the information in that resource. Please also try exporting this document in an incognito/ private browser window and on a different network to rule out any network or browser-related causes. Thanks!