Passing value from one site to another using encrypted query string - 500 - Internal server error

I am trying to pass a value from an external website to our website using encrypted query string and the method on http://www.codeproject.com/Articles/25719/Query-string-encryption-for-ASP-NET?msg=4647192#xx4647192xx. This method runs from Visual Studio on my machine, but when trying to run it on our server, I am getting the error "500 - Internal server error. There is a problem with the resource you are looking for, and it cannot be displayed.". If I took out the below code from web.config, I don't get the error, but it doesn't decrypt the query string. I already include the CryptoKey in the web.config. What causes this error on my server, and how can I fix it ?
<httpModules> <add name="CryptoQueryStringUrlRemapper" type="SmartSoft.QueryStringEncryption.CryptoQueryStringUrlRemapper"/> < ...