I have nop 2.60 installed and have the same error. Whenever I put the plugin folders in the /Plugins folder, I get this error. I have tried so many solutions or workaround you mentioned before but I can't get rid off this error.
I followed the instructions in the link (http://www.nopcommerce.com/boards/t/17455/load-plugins-based-on-their-displayorder-on-startup.aspx)
and I tried copying SevenSpikes.Nop.Core.dll, SevenSpikes.Nop.Framework.dll, SevenSpikes.Nop.Services.dll into the "SevenSpikes.Nop.Plugins.CategoriesHeaderMenu" folder (this folder throws exception as others do)
Can you suggest me another solution ?