If you are promptting login dialog, it might be caused by the IE cannot use the current credential to access the mail box. Please follow these steps to fix it:
- Open IE
- Click the menu Tools > Internet Options...
- Go to "Security" tab
- Select "Intranet sites" or "rusted sites" zone
- Add the OWA to the zone.
- Click OK to apply.
- Click "Custom Level"
- Check the item "Auto logon with current username andd password"
- Click "OK" to apply.