Azure Portal sometimes fails to load. It happened to me several times. It was always caused by exceeding the quota of HTML5 Local Storage. The solution is as simple as clearing the storage and navigate to portal again. Microsoft Edge contains hidden Developer Tools which helps with tasks like this. You can also delete entire browsing history but it will affect all websites, not just Azure Portal.

To fix this issue, follow steps below:
- press F12
- navigate to Debugger tab
- make sure the folder icon below play button is selected
- expand Local storage and select portal.azure.com
- select the first item and hold Delete key until all values are removed
- type portal.azure.com to the address bar again
The error page should contain a fourth option which would delete all cookies and Local Storage content and navigate user back.