Qt Network does not work with multiple IPs on one interface / static routes

My computer has kind of a special network setup, it has a network interface eth0, with a virtual interface eth0:1. This computer acts as a router between eth0 (10.0.1.0/24) and eth0:1 (10.0.2.0/24).

When I start ANY Qt-based application (originally found the issue with the QtWebBrowser plugin for OBS Studio, but I could also reproduce it using the official HTTP Example (http://doc.qt.io/qt-5/qtnetwork-http-example.html)), I can only access the local computer. HTTP requests to localhost (127.x.x.x) are fine, however, a request to any other IP runs into a timeout infinitely.

I don't know my QT version, so I just picked 5.6.0.
I compiled the HTTP example using "qmake; make" and tested with "./http" and it failed.