In your Post you mentioned your are getting status:500,“internal server error”. As mentioned by other this is the Erro caused by theReceiving System. But there is some thing on webMethods Side also.
First are you converting the XML suing escape characters before doing the Http post.
What type of Encoding are u using. Did you used any other type of encoding to resolve the problem.
and If your are using this Http post in a SOAP call check which version is the webMethods SOAP and the receiving SOAP. this may also cause the error.
And just check once for a fix on advantage site. I remember there is one fix post regarding this error