Getting the "The default Report Server Configuration ID could not be found in the SRSServers table." also knows as the @SYS319407 label id.
Well, one possible reasons for this, not counting the rest of the obvious ones, like you really do not have a configuration entered at all, is overlooking that you are working in a clustered AOS topology and of course you have forgot to add a configuration for one of the AOS servers (meaning that you have added a record in the SSRS setup form and chose that AOS name for it in the AOS information fast tab).
For easy reference of the code raising this error, the classes in question are the SRSConfiguration and SRSFrameworkServiceHelper. If you look at the method getReportServerConfigurations in class SRSFrameworkServiceHelper, there is were it does a select like:
select firstonly * from reportServer where
reportServer.IsDefaultReportLibraryServer == true &&
reportServer.aosId == aosName;