Is your data properly encoded in XML safely? I would do a check by taking the XML document you're posting, keeping all the nodes intact, but replace all the data with simple like text. For example, replace every text node and attribute value to xxxxxxx.
Try submitting that. Also, it could be possible that your XML post is being too restrictive for your web server to handle. You could change that.
Is there a way you could post the XML doc here so we can see?