Problem: After upgrading a CRM environment from 8.2 to 9.1 we had the problem, that our outlook app showed the users the error: „Sorry, something went wrong while initializing the app.“
Solution: The securityrole of the users needs the new „model-driven App“-privileg under „Customizations“ –> „Model-driven App“ –> „Read“
More information: You can find more necessary information about required privileges of the outlook app here: https://docs.microsoft.com/en-us/dynamics365/customer-engagement/outlook-app/deploy-dynamics-365-app-for-outlook#provide-security-role-access