Upon starting Visual Studio with the .NET Reflector add-in configured, the following or similar error may occur:

Request for the permission of type 'System.Security.Permissions.FileIOPermission, mscorlib, version=4.0.0.0, Culture = neutral, PublicKeyToken b77a5c561934e089 failed.

This may happen when you had already used the Visual Studio integration option in the stand-alone distribution of .NET Reflector version 6.0, and subsequently upgraded to the Professional edition and configured VS integration using the product installer.

In order to fix this problem, it may be necessary to start the stand-alone version of Reflector, and upgrade using Help->Check for Updates on the menu.

After the upgrade, start Visual Studio and go to Tools->Add-in Manager on the menu, and enable the .Net Reflector addin, ensuring it is also set to run on startup, then close and re-open Visual Studio.