Unable to open SQL reporting services URL http://servername/ReportServer, Failed to initialize the AppDomain:ReportServer_MSSQLSERVER_0

To resolve this issue,
1. Check whether this registry key exists: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETFramework\OnlyUseLatestCLR

2. If OnlyUserLatestCLR entry exists, delete this key and reboot the server.

Reference:
http://social.msdn.microsoft.com/Forums/sqlserver/en-US/efa28eae-dc45-4127-a74d-eb711973703d/failed-to-initialize-the-appdomainreportservermssqlserver0