Follow these tips to troubleshoot your Connected App authentication for Marketing Cloud Connect for Marketing Cloud Engagement.
Users Can't Access Marketing Cloud Connect
Ensure that you granted users the Marketing Cloud for AppExchange User permission and the permission set created during the Connected App Authentication process. Apply this same permission to the Marketing Cloud Connected App.
If those permissions have been granted to the users, check to see if you changed your connected Engagement account or business unit. It’s possible that the system is attempting to log in the users to a previously connected Engagement account or business unit. Salesforce uses tokens stored in a protected custom setting to authenticate between Engagement and other clouds. Resetting your connection doesn’t reset those tokens.
Refreshing the API user connection resets Marketing Cloud Connect users tokens account-wide. Go to the Marketing Cloud Connect configuration page and click Change API User. Reenter the credentials for your existing Marketing Cloud API user.
Reconnect any connected business units after refreshing the user.
Users Receive a Login or Credential Exception When Sending a Marketing Cloud Email
Grant users access to the permission set, created during the Connected App authentication process, to send using Marketing Cloud Connect.