after ~8 hours of investigating the error, it turned out that your android software causing heavy problems handling ordinary POST-Form submits on websites.
While having never problems with GET-submits, POST will fail if i wait more than a couple of seconds, returning the status text:
Request Time-out
Server timeout waiting for the HTTP request from the client.
This is the behaviour on a Galaxy S4 / Stock Browser.
In Opera mobile also "Request Time-out" Error
in Chrome mobile also "Request Time-out" Error
In firefox mobile, the request takes about 15-20 seconds to load the next page everytime i try it - but it succeed.
Immediately after removing the proxy-settings for ADP, all of above described behaviour dissapears.
I could reproduce this with a Nexus 7 Tablet (current android os & browsers) as well as with an older HTC Desire (android 2.2, old browser). Also i noticed same behaviour on different websites (not just those which i'm actually developing).
Needless to say that i've studied my server-configuration as well as response-header codes for almost the whole freaking day...
Could you confirm the problem?