When deploying my custom web pages for an IFD deployment, I got a FileLoadException (Exception from HRESULT: 0x80131401) on accessing the page in the browser.
Basically, this issue arose since anonymous access was somehow disabled for the “ISV/CustomPages” application in IIS while by default it is enabled for the Microsoft Dynamics CRM website.
This implied that my custom web site was using Windows NT Authentication, while the Microsoft Dynamics CRM Website was enabled for anonymous access. Hence, I got “Loading this assembly would produce a different grant set from other instances” on accessing any of my web pages from within MS CRM.
Simply turning on anonymous access for my custom website did the trick and rendered all my custom web pages correctly :)