The modifications I've made are an update from Git following the instructions on the Bitnami wiki, and in apps/discourse/htdocs/config/site_settings.yml, line 616 (developer.force_hostname.hidden.default) I've changed 'default' to false. and then set force_hostname in the admin panel to the hostname of my forum without the port. (Which didn't actually help fix the problem, and I'll probably revert this change.)
I also ran
bnconfig with --server-hostname equal to the proper hostname of the forum without the port, but, this also didn't help.
Otherwise, I haven't modified the install.
The frontend server doing the proxying is Apache HTTPd 2.2.22, the server in the Bitnami stack is the default, and the platform of the host is Debian 7.7 'wheezy', Linux 3.2.0-4-amd64.