Has several preconditions:
1. proxy is configured with a host name, not an IP address
2. QHostInfo returns both IPv4 and IPv6 addresses
3. proxy is only listening on one address (typically the IPv4 address)
4. The address list returned by QHostInfo has the wrong address first (e.g. IPv6 first)

The problem is that the other addresses in the list are not tried if connection is refused on the first address.

Easiest to reproduce by using the test server, and putting the IPv6 address before the IPv4 address in your hosts file.
Run the tst_qnetworkreply autotest, and you'll see many failures.