Recently, one of our client reported an issue where they were trying to export the report to Excel from report viewer window in AX and that was taking forever to complete. After trying different things, I looked at the AOS even logs and found below error message there:

Object Server 01:  An error has occurred in the services framework.  Method: AifMessageInspector::AfterReceiveRequest.  Error: System.ServiceModel.FaultException: Failed to logon to Microsoft Dynamics AX.

So, I added the business connector proxy account as a user in AX and assign System administrator and System user roles to that. You can find the BC user for your AX environment under System services account form. Below is the path for the form:

System administration -> Setup -> System -> System service accounts.