Could not load file or assembly in Release mode

I have a asp.net project in vs 2012. before adding a web reference every thing is OK. but when I add web reference again every things is OK in Debug mode. but in Release mode I have this error.

Could not load file or assembly 'x.dll' or one of its dependencies. Operation is not supported. (Exception from HRESULT: 0x80131515)

when I remove web references it will be OK. but dlls exist in the address. how can I fix this?