SSRS access denied

Nobody can run the SSRS reports I laboriously created, which rather makes life pointless.  Yet they have permissions to do so.  At the highest level.  I upgrade a user to Content Manager.  Nope.  As soon as he tries to run it, it fails. 

I'm indebted to Sajid Pandore for the solution

To see where it was denying the access, he ran this QUERY on the RS1 server

Use ReportServer
Go

select *
from ExecutionLog3
order by TimeStart desc;

It seemed that the query was being denied permission to run one specific dataset, dtsAcYear, which contains data for the Academic Year parameter.  Unfortunately, this is the first parameter the system gets to. 

Checked the permission on that Dataset – we found that the BI User group for some strange reason was not listed there.

I am not sure why it did not inherit the permissions from the higher level.  All the other datasets did!

We added this manually and tested the report all seems to be working now. 

Until the next thing goes wrong