Following the isntall of Update Rollup 2 for Exchange 2010 Service Pack 1, I have found that I was unable to browse to the Outlook WebApp. Receiving an error “Could not find a base theme”.
This may be resolving by running UpdateCAS.ps1 (%ExchangeInstallPath%\Bin\UpdateCAS.ps1) from the EMS. This updates the OWA/ECP virtual directories on the CAS server by coping files from %ExchangeInstallPath%\ClientAccess\owa\Current.
This error appears to occur (from my testing) when Exchange 2010 SP1 Update Rollup 2 is installed without elevated permissions.
Deans Blog 