I actually didn't even think of doing a manual scan since I've tried more than half a dozen different firmwares and the auto scan has always worked without issue. After some investigation I located the information needed to do a manual scan for a single frequency and tried it. To my surprise it worked. It would be a lot more time consuming to scan each frequency manually but since I'm actually only interested in two DVB-T channels (which are not available via satellite) that single frequency scan gave me all I needed.
I still think there's something not quite right with the OpenPLi firmware since it was not able to do the same auto scan that all other firmwares seems to manage. However, now that I was able to get the DVB-T channels I wanted I think I'll keep the OpenPLi firmware.
Thanks for the help! :-)