You may have already signed in to Outlook Web App error when you try to open Outlook Web App in Microsoft 365
Original KB number: 2798988
When you try to open Outlook Web App in Microsoft 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 a Microsoft 365 Outlook Web App session and an Outlook.com-based Outlook Web App session at the same time.
- The Microsoft 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.
- Access the second instance of Outlook Web App by using an InPrivate Browsing session in the browser.
Still need help? Go to Microsoft Community.