Resolution 1: Verify your Dynamics 365 security role includes all of the required privileges documented in the following article:
Resolution 2: Clear your web browser cache.
a. Access Dynamics 365 as a user with the System Administrator security role.
b. Navigate to Settings, Customizations, and then click Customize the System.
c. Expand Entities and click on the Contact entity.
d. Within the Entity Definition section, verify the Primary Image setting is set to Entity Image. If it is set to [None], this can cause this error to occur. Repeat steps b-d for the Lead and User entity. If you had to change this setting for any of the entities, publish the customizations for the changes to take effect.
NOTE: Microsoft plans to release a fix for this issue.
Resolution 4: Check to see if your Dynamics 365 (online) organization is not available or something is blocking communication with Dynamics 365 (online). Verify you can access your Dynamics 365 organization via your web browser. If you cannot access your Dynamics 365 organization via your web browser, the Dynamics 365 App for Outlook will not be able to access your organization either. If you are unable to access your Dynamics 365 organization with your web browser, contact your Dynamics 365 administrator.