I realize the question might be rather old, but you say the backend is running on the same server. That means on a different port, probably other than the default port 80.
I've read that when you use the "connectionManagement" configuration element, you need to specify the port number if it differs from the default 80.
LINK: maxConnection setting may not work even autoConfig = false in ASP.NET
Secondly, if you choose to use the default configuration (address="*") extended with your own backend specific value, you might consider putting the specific value first! Otherwise, if a request is made, the * matches first and the default of 2 connections is taken. Just like when you use the section in web.config.
LINK: <remove> Element for connectionManagement (Network Settings)
Hope it helps someone.