Thanks emmrecs,
I ran MediaInfo on both my files and they are exactly the same, except for the framerate.  The files in question are shown below, named "workingfile.mkv" and "failedfile.mkv":

workingfile.mkb - English, 1920*1024 (1.85:1), at 23.976 fps, AVC (Main @ L4.0)(CABAC / 4 Ref frames)

failedfile.mkv - English, 1920*1024 (1.85:1), at 23.976 fps, AVC (Main @ L4.0)(CABAC / 4 Ref frames)

Both of these files were encoded using the same settings on handbrake.fr - a common video file conversion software.  I looked at the original MOV files used to make these MKV's and the AVC was slightly different:

The working file was encoded from a file that MediaInfo describes as:  AVC(Main @ L4.0)(2 Ref frames)
and the failing file was encoded from a file that MediaInfo describes as:  AVC(Baseline @ L5.0)(1 Ref frames)

I don't think the original file is relevant (both play in MEP MX), but I put it in there just in-case someone thinks it makes a difference.

But at this stage, my problem persists, I have 2 MKV files exactly the same and 1 plays and 1 does not.

OK.

Thanks for the info re handbrake; it's a program I'd never previously heard of.  However, since the files you have problems with are "converted" by this software, but the originals apparently play without difficulty in MEP MX, I wonder why you are carrying out the conversion?

In terms of the info about those original files, I think there may be some clue to their difference in the fact that one is described as being "Main @ L4.0" and the other "Baseline @ L4.0".  I believe this refers to codecs used when those files were created.  I wonder whether handbrake cannot properly deal with the "Baseline" codec data and, in making the conversion, is creating an "error" that MEP cannot handle.

One other piece of information that would be useful in helping to diagnose this is what Audio Sampling Rate is used on each file.  (MediaInfo will tell you this.)  I ask because I know MEP and VPX can both "throw a wobbly" if presented with an audio track which is not at whatever bit rate and sampling frequency it expects, normally 16 bit and 48kHz.

Magix has always been a problem when it comes to mkv's and it seems Magix does not make it a priority in or care about fixing the problem since its been years. As an editor I do get a fair share of mkv's because it does a better job at compressing a file within its container and most mkv files use the .h264 codec which for some reason Magix won't open it or any other mkv file format. Magix will open MOV, MP4, Xvid/avi and some others using .h264 codec but mkv not so much. I really think Magix needs to stop stating it opens mkv's when everyone encounters a problem. Apparently MMEP only opens an old, real old version of mkv no one uses anymore. For a video editor its a pretty descent one but there are quite a few things that they can do to greatly improve the program itself including many of the effects which some are pretty old and no one uses. If I use MMEP I never use it for color grading, the grading in the program is not the greatest, they do have some movie looks but they are not that great either most of them are not good at all, you can't adjust them once applied as well. It would be nice to see that you can add film luts but Magix dont currently have that option. All my grading is done in After Effects, for the most part I edit in Adobe Premier, it has better tools and a better range of effects, but I will use Magix for smaller projects.

...If I use MMEP I never use it for color grading, the grading in the program is not the greatest, they do have some movie looks but they are not that great either most of them are not good at all, you can't adjust them once applied as well. It would be nice to see that you can add film luts but Magix dont currently have that option. All my grading is done in After Effects, for the most part I edit in Adobe Premier, it has better tools and a better range of effects, but I will use Magix for smaller projects.

You are comparing an 8-year old consumer video editing program, Movie Edit Pro Plus/Premium, to Adobe Premier or After Effects? Seriously? (This thread is dated 2012.)

LUTs can be used and created in Magix Video Pro X. VPX also has the normal measurement instruments and much more in the way of colour correction and grading tools than the current Movie Edit ProP.

The following formats are supported in VPX:

AVC-Intra*, AVI, DNxHD, DV-AVI, DVCPRO*, MJPEG, MKV, MPEG-1, MPEG-2, MPEG-4, MOV, MVC, MXF, WMV

*Note: This program requires an Internet connection for many of its functions. MP3 export requires installation of Windows Media Player Version 10 or higher.
DVCPRO, AVC-Intra and HEVC encoding require fee-based activation.

There may be some MKV's that cannot be opened in VPX. Someone always manages to find a program or a way to create incompatible material.

John CB