Can't open most mkv since version 2.6.12

[18Phoenix]

Hi 
Since version 2.6.12 (+13) I can't use the most of my mkv's.
Message: Attempt to open x.mkv failed ! Could not open the file.
If I reinstall Avidemux 2.6.11, everything works always fine.
But no more with 2.6.12 + 2.6.13.
If I remux the file with MKVToolNix without changing anything, then it works again.
Strange.
Is the problem known ?

[mean]

nope
A sample would help

[18Phoenix]

I can't give a sample because of the big size of the files.
When I cut out a part, it works again, so no small sample as a result.
I've fixed the problem with running all mkv through MKVToolNIX without any chances.
Strange, veeeeeery strange.
But with 2.6.11, every mkv has been acceptet.
Now they work again with 2.6.13, but I don't understand the difference.
Must has something to do with the header, because I didn't convert video and audio.
But I haven't seen a difference with mkvinfo-gui.