Looking at your error from the screenshot it seems that it is trying to load an assembly and it doesn't have enough rights. I had exactly the same problem when adding some assemblies in the bin folder of the sharepoint site. The problem didn't exist on my dev environment because the security policy was set to Full, but when I moved it on a production environment I was not able to load my custom pages because the policy didn't allowed it. Can you please check what is the value in your policy setting in the web.config file of your sharepoint site (<trust level="WSS_Minimal" originUrl="" />
) - if it is WSS_Minimal or WSS_Medium you can try to change it to Full and try it again. If this fixes it, this is your problem and you may have to create a custom policy file that will give the needed rights to the assembly. Can you please tell me what is the assembly it is trying to load. Your screenshot is not giving me this information. Also if the assembly is put into GAC this will also resolve it, but I peresonally prefered to have some more control.
Here is some more info if you want to dig more into it:
http://msdn.microsoft.com/en-us/library/ms916855.aspx