This same behavior is happening to me. Zotac ZBOX, OpenElec with TVHeadend 2.1.6. Although, it looks like the version doesn't matter. It craps out around 999000000 in the frequency. I'm guessing this has to do with what's been allocated... since most PVR software is written for countries other than the US, is it possible that QAM-256 frequencies exceed those used for DVB? I'm just taking a shot in the dark.
I'm watching the debug log, but it's given up. I have 24 muxes left to scan, so there's no chance I'll ever get to 0 and complete. Without completion, I'm not going to get to map these services to channels. That pretty much disqualifies me from getting to use this software.