Most folks with this problem notice two things:

1) Run the browser or SQL Data Tools as administrator and permissions are not a problem.

2) Once Reporting Site Settings are accessed from a browser started as administrator, one can add the domain\user with failing permissions to solve the problem.

In my case, number 1 is true but number 2 is not.

I add 'myComp\myUser' giving them System Administrator roles from within SSRS Site settings but still have the same problem.

User 'myComp\myUser' does not have required permissions. Verify that sufficient permissions have been granted and Windows User Account Control (UAC) restrictions have been addressed

Any ideas what I'm missing?

J