SSRS 2005 Issue: 'Method not Found: CleanCurrentUserName()' and 'Report Server is not responding'

I've been researching an issue today relating to a problem with SSRS 2005, the symptoms are as follows;

1. "Report server is not responding" error in the browser when connecting to the Report Manager website.
2. The following message in an ASP.NET entry in the application event log on the server:

     Exception information:
        Exception type: MissingMethodException
        Exception message: Method not found: 'Void Microsoft.ReportingServices.Diagnostics.UserUtil.CleanCurrentUserName()'.

It this case the cause was a mismatch in the ReportingServicesWebServer.dll assembly version, for some reason the assembly hadn't been updated when the service pack was applied and the assembly was an older version than ReportingServicesService.exe.  Looking through our internal KB it seems that the recommendation was to destroy the installation of SQL Server as a whole and reinstall, in a few cases that didn't solve it either.  Becuase it seems that the only option provided was a reinstall I felt a risky manouver was worth trying and it turned out to be a success. 

Backup then delete the offending assembly then apply the latest cumulative update, a reboot may be required.  

The assembly is replaced as a result of the CU update and all the assemblies are brought up to the same version, it should be as easy as that and certainly a lot quicker than a resinstallation!

Please note, this may still not resolve your issue but if it does it will save you many hours of reinstallation.  Please ensure you perform full backups of your databases on the off chance it makes things worse and bare in mind that the logic inside the CU updater itself may change in the future.  That said, if it does work feel free to drop me a comment as confirmation. :)

Regards,
Jason