NAV 2013 R2 on Windows Server 2012 R2 and .NET Framework

Hi,

We have NAV 2013 R2 installation on three separate machines. (SQL Server, NAV Server, Web Server)
All three machines use Windows Server 2012 R2 Standard.
Only .NET Framework 4.5 is installed on all three servers.

When "Network Service" user is used for NAV Server Service, everything works fine, but if we use a dedicated User (SPN set, log as service, ...) to run NAV Server Service we get following error message

Type: System.IO.DirectoryNotFoundException
Message: The path to Windows Communication Foundation was not found on the service tier. It was expected to be in folder 'C:\Windows\Microsoft.NET\Framework64\v3.0\Windows Communication Foundation'. Have you installed the .NET Framework 3.0?
StackTrace:
     at Microsoft.Dynamics.Nav.Runtime.CSharpCompiler..ctor()
     at Microsoft.Dynamics.Nav.Runtime.NCLMetadata..ctor(NavDatabase ownerDatabase)
     at Microsoft.Dynamics.Nav.Runtime.NavTenantCollection.AddSystemTenant(String appDatabaseServerName, String appDatabaseName)
     at Microsoft.Dynamics.Nav.Runtime.NavTenantCollection.AddSingleLegacyTenant(String databaseServerName, String databaseUserName, String databaseUserPassword, String databaseName)
     at Microsoft.Dynamics.Nav.Runtime.NavTenantCollection.ConfigureTenants(ServerUserSettings settings)
     at System.Threading.Tasks.Task.Execute()
Source: Microsoft.Dynamics.Nav.Ncl
HResult: -2147024893

Which is okay as .NET 3.0 is not installed.
But as per System requirements for NAV 2013 R2 it says that only .NET 4.5 is needed.
Also why is NAV code switching back to .NET 3.5 when a dedicated user is used to run NAV Server and not for "Network Service" ?!?!

Thanks for your help.