Beside the configuration of the univention-proxy.conf (see Bug #44524), the behaviour of the univention portal site is not "reverse proxy friendly". In my tests trying to access a UCS server with an "internal only" FQDN using from the internet using a reverse proxy or port forwarding, I ended in a loop error in firefox (too many redirects).
I cannot reproduce this. I set up a reverse proxy with the following configuration on my local computer: Listen 90 <VirtualHost *:90> ProxyPass / http://xen3.school.local/ </VirtualHost> and accessed http://localhost:90/ via firefox. The portal and UMC works without problems.
Re-tested with UCS 4.2 Errata 139, no re-direct loops anymore.
Thanks!