What would cause a 500 Internal Server Error when accessing the Report Manager url in SQL Server Reporting Services 2008 R2?

I'm very new to Reporting Services and I'm not even sure what to ask. I was given a server where it was installed. I can run the Reporting Services Configuration Manager and connect. However when I attempt to access the urls (web service and report manager) I get a 500 error. I don't see anything in the event viewer. Are there log files somewhere? I'm suspicious of the IIS setup. In particular, the virtual directories. How should these look? I know this question is broad, but general guidance is appreciated.

Update: After examining the logs, here is the first error:

servicecontroller!WindowsService_0!358!09/07/2010-13:30:52:: e ERROR: Exception caught loading and setting code permissions policy level: System.NotSupportedException: This method explicitly uses CAS policy, which has been obsoleted by the .NET Framework. In order to enable CAS policy for compatibility reasons, please use the NetFx40_LegacySecurityPolicy configuration switch. Please see http://go.microsoft.com/fwlink/?LinkID=155570 for more information.
   at System.AppDomain.SetAppDomainPolicy(PolicyLevel domainPolicy)
   at Microsoft.ReportingServices.Library.ServiceController.SetAppDomainPolicy()
library!WindowsService_0!358!09/07/2010-13:30:52:: e ERROR: ServiceStartThread: Exception caught while starting service. Error: System.NotSupportedException: This method explicitly uses CAS policy, which has been obsoleted by the .NET Framework. In order to enable CAS policy for compatibility reasons, please use the NetFx40_LegacySecurityPolicy configuration switch. Please see http://go.microsoft.com/fwlink/?LinkID=155570 for more information.
   at System.AppDomain.SetAppDomainPolicy(PolicyLevel domainPolicy)
   at Microsoft.ReportingServices.Library.ServiceController.SetAppDomainPolicy()
   at Microsoft.ReportingServices.Library.ServiceController.ServiceStartThread(Object firstStart)

After fixing the CAS issue, I now find this error in the log (after my initial attempt to access the web service):

appdomainmanager!DefaultDomain!f6c!09/07/2010-14:32:20:: e ERROR: AppDomain ReportServer_11 failed to start. Error: The configuration system has already been initialized.
library!DefaultDomain!f6c!09/07/2010-14:32:20:: e ERROR: Throwing Microsoft.ReportingServices.Diagnostics.Utilities.ReportServerHttpRuntimeInternalException: Failed to create HTTP Runtime, Microsoft.ReportingServices.Diagnostics.Utilities.ReportServerHttpRuntimeInternalException: An internal or system error occurred in the HTTP Runtime object for application domain ReportServer_11.  ---> System.InvalidOperationException: The configuration system has already been initialized.
   at System.Configuration.ConfigurationManager.SetConfigurationSystem(IInternalConfigSystem configSystem, Boolean initComplete)
   at System.Web.Configuration.HttpConfigurationSystem.EnsureInit(IConfigMapPath configMapPath, Boolean listenToFileChanges, Boolean initComplete)
   at System.Web.Hosting.HostingEnvironment.Initialize(ApplicationManager appManager, IApplicationHost appHost, IConfigMapPathFactory configMapPathFactory, HostingEnvironmentParameters hostingParameters, PolicyLevel policyLevel, Exception appDomainCreationException)
   at System.Web.Hosting.HostingEnvironment.Initialize(ApplicationManager appManager, IApplicationHost appHost, IConfigMapPathFactory configMapPathFactory, HostingEnvironmentParameters hostingParameters, PolicyLevel policyLevel, Exception appDomainCreationException)
   at System.Web.Hosting.ApplicationManager.CreateAppDomainWithHostingEnvironment(String appId, IApplicationHost appHost, HostingEnvironmentParameters hostingParameters)
   at System.Web.Hosting.ApplicationManager.CreateAppDomainWithHostingEnvironmentAndReportErrors(String appId, IApplicationHost appHost, HostingEnvironmentParameters hostingParameters)
   at System.Web.Hosting.ApplicationManager.GetAppDomainWithHostingEnvironment(String appId, IApplicationHost appHost, HostingEnvironmentParameters hostingParameters)
   at System.Web.Hosting.ApplicationManager.CreateObjectInternal(String appId, Type type, IApplicationHost appHost, Boolean failIfExists, HostingEnvironmentParameters hostingParameters)
   at System.Web.Hosting.ApplicationManager.CreateObject(String appId, Type type, String virtualPath, String physicalPath, Boolean failIfExists, Boolean throwOnError)
   at ReportingServicesHttpRuntime.RsHttpRuntime.Create(RsAppDomainType type, String vdir, String pdir, Int32& domainId)
   --- End of inner exception stack trace ---;

From serverfault bennage

• SSRS does not rely on IIS (from SQL 2008 onwards), it uses his own http.sys enviroment to host the web service and the report manager.

  Refer to:

  C:\Program Files\Microsoft SQL Server\MSRS10_50\Reporting Services\LogFiles
  

  To the possible cause.

  With so little detail it is hard to know, If you come up with an error in the logfiles, post it to help further.

  From Mikeware

• In the end, I installed on a server without .NET 4.0 and I was able to get it working.

  From bennage