WIth Server 2008 R2, Plesk 9.5 has a bad habbit of not giving itself permissions to the C:\Windows\Assemblies folder. Due to this, ASP and ASP.NET applications will not function correctly. In order to fix this you must follow the steps below.

With Windows Explorer, browse to C:\Program Files (x86)\Parallels\Plesk\etc\DiskSecurity\DiskSecurity.xml
Right click on DiskSecurity.xml and edit the file with Notepad or another plain text editor.
Insert the following line at the bottom of the file, BEFORE </Entries>:

<