Wsus 3.0 Install Error: Request for principal permission failed

System is a win2003 server/en with sp2 and latest patches installed. Database
is a Sql 2005 ExpressEdition/SP1/en. We use McAfee 8.5 Enterprise -
deactivated at install time. Install went fine until it configures IIS
dynamic compression. Detailed error:

The transacted install has completed.
StartServer starting...
Generating encryption key to write to the registry...
Generating encryption key to write to the database...
Generation of encryption key to save to the database failed.
Error=System.Security.SecurityException: Request for principal permission
failed.
at System.Security.Permissions.PrincipalPermission.Th rowSecurityException()
at System.Security.Permissions.PrincipalPermission.De mand()
at System.Security.PermissionSet.DemandNonCAS()
at
Microsoft.UpdateServices.Internal.DatabaseAccess.A dminDataAccess.ExecuteSPSetConfiguration(Configura tionTableRow
row, IDictionary changedLanguages, IEnumerable`1 programKeys)
at
Microsoft.UpdateServices.Internal.BaseApi.UpdateSe rverConfiguration.Save(Boolean detectConfigChange)
at
Microsoft.UpdateServices.Internal.BaseApi.UpdateSe rverConfiguration.Save()
at
Microsoft.UpdateServices.Setup.StartServer.StartSe rver.GenerateNewDatabaseEncryptionKey()
The action that failed was:
Demand
The type of the first permission that failed was:
System.Security.Permissions.PrincipalPermission
The first permission that failed was:
<IPermission class="System.Security.Permissions.PrincipalPermis sion,
mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089"
version="1">
<Identity Authenticated="true"
Role="ChangeServerSetting"/>
</IPermission>

The demand was for:
<IPermission class="System.Security.Permissions.PrincipalPermis sion,
mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089"
version="1">
<Identity Authenticated="true"
Role="ChangeServerSetting"/>
</IPermission>

The assembly or AppDomain that failed was:
mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089
StartServer encountered errors. Exception=Request for principal permission
failed.

I have not found anything in the web nor in groups regarding this error.
Maybe there is somebody who knows...

Torsten