SQL server report error - "rsInternalError"

Question

When I try to access the Report Server (or open up a report), I get the following error,

An internal error occurred on the report server. 
See the error log for more details. (rsInternalError)
For more information about this error navigate to the report server 
on the local server machine, or enable remote errors

When I checked the server, both "SQL Server (MSSQLSERVER)" and "SQL Server Reporting Services (MSSQLSERVER)" services are running.

Could not figure out any issue and I have restarted both the services. Then reports started working. Actually restarting the SSRS service is enough it seems. After few hours, this issue happened again, I have restarted both the services, it started working again.

We are using SQL Server 2008 R2

Reportserver logfile, stacktrace

   --- End of inner exception stack trace ---
library!ReportServer_0-3!1e54!11/12/2013-17:38:06:: e ERROR: Throwing Microsoft.ReportingServices.Library.ReportServerDatabaseUnavailableException: , Microsoft.ReportingServices.Library.ReportServerDatabaseUnavailableException: The report server cannot open a connection to the report server database. A connection to the database is required for all requests and processing. ---> System.IO.FileLoadException: Could not load file or assembly 'Microsoft.ReportingServices.UpgradeScripts, Version=10.0.0.0, Culture=neutral, PublicKeyToken=89845dcd8080cc91' or one of its dependencies. Access is denied.
File name: 'Microsoft.ReportingServices.UpgradeScripts, Version=10.0.0.0, Culture=neutral, PublicKeyToken=89845dcd8080cc91'
   at Microsoft.ReportingServices.Library.ConnectionManager.get_ReportingServicesVersionNumber()
   at Microsoft.ReportingServices.Library.ConnectionManager.IsUpToDateDatabaseVersion(String storedDBVersion)
   at Microsoft.ReportingServices.Library.ConnectionManager.EnsureCorrectDBVersion()
   at Microsoft.ReportingServices.Library.ConnectionManager.ConnectStorage()
   at Microsoft.ReportingServices.Library.RunningJobsDb.GetMyRunningJobs()

WRN: Assembly binding logging is turned OFF.
To enable assembly bind failure logging, set the registry value [HKLM\Software\Microsoft\Fusion!EnableLog] (DWORD) to 1.
Note: There is some performance penalty associated with assembly bind failure logging.
To turn this feature off, remove the registry value [HKLM\Software\Microsoft\Fusion!EnableLog].

   --- End of inner exception stack trace ---;

What can be the issue?

Solution

There was a pending Windows Update. I have restarted the server to install the updates. After that the issue did not occur. Seems like it's related to Windows update process.