Error 104: Connection reset by peer with ASP.NET MVC app

I'm having a problem with a MVC (1.0) app that I can't figure out at all. There's two versions of the site (live and UAT) hosted on the same server. For each version of the site, the same code is shared by multiple organisations who each have their own database (MSSQL2005) and a separate web site in IIS (7.5) (pointed to the same code).

The UAT site has an update to the code and the database that is waiting to be deployed to the live site.

One of the customers ("customer A") is getting an error "104: Connection reset by peer" when they try to log in to the UAT site. They can see the login page but when they submit their login details the connection seems to be timing out (the requests seem to take ~130s to complete).

Customer A can log in fine to the live site. The other customers don't have a problem logging into the UAT site or the live site. If I try to log in as customer A, using their login details, it all works fine from within our network, and also from outside our network.

Customer A seems to be using squid as a proxy.

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I can't think what the problem could be, and I've run out of ideas of things to test. The fact that I can log in as the customer fine and other customers don't have any issues seems to eliminate the code and database as problems.

What other things could I do to try and isolate the problem?

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By dumping out the request data I was able to work out that something (I'm guessing the proxy) was removing the form values from the request. This obviously meant the app didn't work properly.

However, it seems whatever was removing the form data was leaving content-length unchanged, which would explain why the client was timing out waiting for more data and the server thought it was finished.

By using https instead of http (which we were going to do anyway), the request tampering seem to have stopped.