1. Make sure you can login via IE
2. Assume you are using O365 CRM, in the URL place you should type
https://disco.crm5.dynamics.com or select the CRM Online from the Dropdown.
3. Make sure the date/time and time zone is accurate.
Time difference no more than 1 minute
4. Upgrade your Outlook client to at least UR12.
Hope this helps
Wilson