The only thing I can offer here is the following Help link that may provide a bit more info for you. I honestly cannot recall if I have heard of anyone doing reverse proxy setup with our web server as well, so it may be better to switch over to IIS/QlikView Settings service instead as well if you know IIS better. Our QlikView Web Server is really designed for environments where it is a small environment where there is not much in the way of IT resources available, but it is based upon the IIS engine, just FYI.
I do believe your theory is on the right track, but I am no guru here, so I am not sure if there is a means to overcome things or not. The only thing of which I can think is the proxy is causing things in our backend to think the connection has changed and is invalidating the ticket we generated, but that is one thing to check, look in the QVS event log and see if there is a ticket being generated upon your Auth request. Now that I think about it, you may need to look in the QVWS log in this case, as I think you will see things there too.
Apologies I do not have anything better but hopefully this may help a little. Try Fiddler trace as well, as sometimes that can provide some insights or maybe just the web browser dev tools network trace as well potentially may catch something you cannot see directly.
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