When you try to open Outlook Web App in Microsoft Office 365, you receive the following error message:
There was a problem opening your mailbox. You may have already signed in to Outlook Web App on a different browser tab. If so, close this tab and return to the other tab. If that doesn't work, you can try:
- Closing your browser window and signing in again.
- Deleting cookies from your browser and signing in again.
This issue occurs when the following conditions are true:
You try to open an Office 365 Outlook Web App session and an Outlook.com-based Outlook Web App session at the same time.
The Office 365 Outlook Web App session and the Outlook.com-based Outlook Web App session share the same Outlook Web App server endpoint.
The same Outlook Web App server endpoint can't be accessed through a single browser session.
Use one of the following methods, as appropriate for your situation.
Set up CNAME records in the external Domain Name System (DNS) server, and set up custom Outlook Web App URLs. For more information about how to do this, see page 19 of the following document: