原因是修改了wcf webservice的方法，需要把.svc文件覆盖原来的即可解决。详见下网页。
Could not load file or assembly App_web_.... (WCF) - ASP.NET Forums
It could be that whenever you change method signature (Add / remove methods, change parameters. change method names etc.) it may happend.
You need, whenever you deploey the service class'es DLL, to copy the .svc file as well.
