I just got a new router set up for gigabit internet and I'm still getting used to the new settings, but one thing I've noticed by looking at the UPnP/NATPMP diagnostics log is how Tixati will only ask my router on port 1900 (and failing), but another app I've used called Portmapper connects using port 5000 successfully. Other programs like qBittorrent also doesn't seem to have a problem connecting to my router and open ports using UPnP, so I'm wondering if my router only accepts UPnP on port 5000 and not 1900. It's not a firewall thing, I've tried with that disabled as well.  
A shame there are no configuration file I can change that it.

In fact, turns out that while I was suspecting Tixati of not doing UPnP "correctly" it was (big surprise) my ISP that has purposefully gimped the modem they've put up in my flat. UPnP doesn't work, port forwarding neither. So this thread is already dead in the water. If anyone is curious, the modem in question is a Icotera i5850 from Danish ISP "Fibia" (also called "Waoo").