Content Length Header Already Present

Recently upgraded to 4.5.1 and imported my project. A REST request that was previously working is now returning a "Content length header is already present" header. I've played around with the global settings to no avail.

It worked once when I checked the box "Authenticate Preemptively" in HTTP settings, then stopped working again the second time I tried it.

The actual content of the request is as follows:
POST http://www.myurl.com/createFeed HTTP/1.1
Accept-Encoding: gzip,deflate
Accept: text/xml
Content-Type: application/xml
Content-Length: 443
Host: www.myurl.com
Connection: Keep-Alive
User-Agent: Apache-HttpClient/4.1.1 (java 1.5)
<xml> XML HERE </xml>


Any ideas on how to trouble shoot this? I've come across very little online and nothing in the tutorials???

Thanks!