Jul 19, 2016 06:17 AM|Yohann Lu|LINK
The remote server returned an error: (500) Internal Server Error
When the remote server returns an error, then it means that the remote server noticed something bad. When the error is 500, that means it's an internal error, meaning internal to the service - the service threw an exception that was not caught.
So, you can try the following suggestions.
1: Look in the Windows event logs on the service to see what went wrong.
2: Are you sending the proper XML message (SOAP)? If your message isn't formatted properly, you will definitely get a server 500 error back.
3: You could use the Fiddler. In my experience is the best tools to check what is going on behind the scene.
4: Invoking Web Service dynamically uses HttpWebRequest :
5: Refer the following post solution. May be it will helpful for you.
ASMX web service and The remote server returned an error: (500) Internal Server Error issue: