Today out of the blue I received this error for SQL Reporting Services on SCCM 2012.

Service cannot be started. System.Exception: Default appdomain failed to initialize.
at Microsoft.ReportingServices.Library.ServiceAppDomainController.Start()
at Microsoft.ReportingServices.Library.ReportService.OnStart(String[] args)
at System.ServiceProcess.ServiceBase.ServiceQueuedMainCallback(Object state)

When you try and start the Reporting Services it gives a warning saying some services start and then stop…….

My suspicion was that SQL was running on a trial version but Event Viewer was not giving me that info. I restarted the server and after logging in I then got the error that it had. Ran the upgrade for SQL to Standard and the service started.

Hope it helps