Did you install Magick.NET-x64 in the GAC on your server? I don't have experience with installing assemblies in the global assembly cache but it looks like your application cannot find Magick.NET-x64 in the GAC. Maybe you should add the following to your web.config:
<system.web><compilation><assemblies><addassembly="Magick.NET-x64, Version=6.0.0.0"PublicKeyToken="2004825badfa91ec"/></assemblies></compilation></system.web>