Had a problm with ASP.NET applications generating an Access Denied exception when trying access system.servicemodel.

Turns out this is a problem with Plesk on Windows, and fortunately has a simple fix.

Log in as Administrator and execute the following:

cacls C:\Windows\assembly\GAC_MSIL /E /R psacln /T /C
cacls C:\Windows\assembly\GAC_MSIL /E /R psaadm /T /C
This came from this KB article and fixed the problem for me anyway.