Sorry to necro an old post, but in the interest of improving internet search results for this particular problem, here's what I did to fix the OP's problem (I had the same thing):
There are a number of places I found with the information in the following link:
In my specific case, the RsExecRole role in both the "master" and "msdb" databases had had both the "NT Authority\Network Service" and "NTAuthority\System" logins removed from its members (the RSExecRole had had all its members removed by someone doing some maintenance). I compared another Reporting Services installation on another server after reading information in the above link.
1. In Mangement Studio connect to the SQL Server instance on which Reporting Services runs.
2. Databases > system Databases
3. master > Security > Roles > RSExecRole (then add whatever logins are required... check in the ReportServer or ReportServerTempDB databases if you're not sure)
4. msdb > Security > Roles > RSExecRole (add the same logins you did to the master DB)
In the end you need the same logins in RSExecRole role for the master, msdb, ReportServer, and ReportServerTempDB databases. ("ReportServer" is the default Reporting Services database name, in case that's not clear