(500) Internal Server Error could be any number of things. Unfortunately the response shown there doesn't provide much in the way of what's actually causing the error. Try setting a breakpoint on the first line in RTShipping.POSTandReceiveData
Code:
private string POSTandReceiveData(string Request, string Server)
{
// check for cache hit:
String CacheName = Server + Request; // breakpoint this line
and step through until you get an error. The exception here should provide some insight into what the problem is.
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