CRM outlook client
When accessing Microsoft Dynamics CRM records within the Microsoft Dynamics CRM for Microsoft Office Outlook client, a user may experience intermittent prompts for credentials.
The issue is due to an Internet Explorer problem which caused session cookies to be cleared out and require users to log in again.
The Internet Explorer team has addressed this issue as a hotfix which can be found in KB 2926827:
This issue affects Internet Explorer 10 and 11.