This can occur for 2 reasons:
1. You are attempting to login with user credentials that do not have permissions to access this page
This issue occurs after attempting to log into the Administration portal (clicking 'Log In with Microsoft CRM') you are shown a blank page.
If your consoles look like the above, you are experiencing this issue. To access the console logs right click anywhere on the blank page and select "inspect" then in the developer tools panel that appears select the "console" tab.
This issue occurs when you are already logged in with a Microsoft account that does not have permissions to access this page, such as with a dynamics account that is not configured for Live Assist, or any account in the Microsoft Family, such as a Hotmail account or a Office365/Live.com account. The administration portal does not provide the ability to choose which account is used if multiple accounts are logged in, so it selects the default account that has a live session.
Please login using an account with the correct permissions using an Incognito window, or by clearing your cache or restarting your browser to resolve this issue.
2. Your URL is pointing is pointing at an Expired Live Assist Account ID
If you have an Organization ID in the URL and you see an error similar to the following in the console logs:
EXCEPTION: Uncaught (in promise): Error: Cannot match any routes: 'df5babdb-7480-4272-b925-8b9479b92157/f448e060-3702-4e2f-b2d4-39d697b22cda'
You are trying to login using a URL pointing to a CRM instance with an expired Live Assist account. This happens when the browser has cached URLs for now expired test accounts.
Removing the text in the URL to the right of liveassistfor365.com/portal/ and login back in should resolve the issue. Clearing the browser cache will stop the wrong URL being used again.