www.makemkv.com

Maybe the DTS HD Master or TrueHD tracks which are unsupported? Don't know if the Xbox playback software supports it. All others looks okay.

Don't unterstand why an HD audio track is needed to playback something on the Xbox. To hear the addional "HD" details, you must have 10K$+ equipment with 5/7 or more very good speakers.

I've got a bunch of older rips with DTS-HD Master as the default audio track that work just fine.

Is there any fix to this problem now?
Ive got the exact same issue...Some MKV Files wont work, even if some others with EXACTLY the same Codecs (Video and Audio) work like charme.
Best example: Ive got a File which isnt working, but the sample file works! So I think this has nothing to do with the codecs themselves.
And yes, all the files are playable on my Win PC in VLC without any problems...