Problem connecting to SSRS reporting services
I have the same problem except my Reporting Service is 2008 R2.

Anyway I have SSMS 2014 on the server where Reporting Service is installed and get the same error.

I have installed SSMS 2008 R2 and straight away it works.

So you must have same version SSMS as Reporting service.

And here is explanation:

In Reporting Services, the namespace for SSRS 2014 is root\Microsoft\SqlServer\ReportServer\RS_\v12\Admin
while the namespace for SSRS 2012 is
 root\Microsoft\SqlServer\ReportServer\RS_\v11\Admin.