This intermittent problem seems to happen when IE, NTLM and probably a proxy is involved:
"From time-to-time, web developers contact the IE team reporting that they’ve encountered a problem whereby Internet Explorer submits a POST but fails to transmit the content body. This bodyless POST indicates via the Content-Length header that the POST is zero-bytes long, regardless of how much data was supposed to be uploaded.
This behavior occurs on servers that use challenge-response authentication protocols like NTLM or Kerberos/Negotiate. For performance reasons, Internet Explorer does not immediately transmit a POST body if it expects to immediately receive a HTTP/401 response with a challenge for credentials. Without this optimization, IE would be forced to reupload the (potentially large) POST body multiple times, wasting bandwidth."
This problem is described here:Challenge-Response Authentication and Zero-Length Posts
If you are using ISA Server 2006 as a proxy, please apply the fix here:POST requests that do not have a POST body may be sent to a Web server that is published in ISA Server 2006