In a hybrid Exchange environment, you may get an error when attempting to log in to Office 365 when performing a management task involving Office 365. When clicking the "Sign in to Office 365" link, the following error is displayed:

critical error

The web page isn't loading correctly. Please reload the page by refreshing your browser.

Technical information: Unable to set the property "mkt" of a null reference or undefined

Not really a solution, but you can work around the issue by performing these steps:

1. In EAC, navigate to Hybrid.
2. Click Modify.
3. Click Sign in to Office 365, type in your Office 365 username and password.
4. Once logged in, click on the Enterprise tab and try performing the management task (e.g. a migration) again. You should not get the sign in prompt and you should be able to complete the migration.