CRM 2016 Reporting Configuration

After installing SSRS Data Connector on SQL2014 Server named instance as Reports user (has the rights to the databases - config, CRM, Reports, local admin on SQL server) , I get this infamous error "Error occurred while fetching the data extension".

The reporting server URL is correct, I can browse all of the default reports. I granted the Reports account Sysadmin and publisher right inside of SSRS site but  still no luck. 

I have a few questions.

1. Which account should be used to run SSRS Windows service?

2. Which account should be used as service account inside of SSRS Configuration page?  

Any advice is appreciated.