Muxer - cannot open

[thany]

Getting this error again:

Muxer
Cannot open

Just after the first pass of a two-pass encode. The input is a WMV file, with 1920x1080 WMV3 video @ 29.97fps, and WMA stereo audio @ 192Kbps. A filter is applied, swsResize to bring it down to 1280x720. The output is MKV with MPEG4 AVC with all settings default except for Encoding mode = Average bitrate & Average bitrate = 2500. All other settings are left untouched. Audio is Vorbis, all settings are left at the default.

The input file is something as simple D:\04.wmv and the output file is D:\04.mkv, just as simple.
I have other files with exactly the same properties, treated exactly the same, yet they work. I have no idea what the difference between them is.

This is Avidemux r9034 running on Windows XP SP3.

I don't understand what this message is trying to say. A little bit more info in this message would help, i.e. what exactly is it trying to do that doesn't work right. Iow, the message is too sparse.

What should I do to fix or diagnose this error?

[mean]

Maybe : Not sure wma in mkv is allowed by the muxer

[thany]

Sorry, maybe I didn't clear that up properly. I was trying to encode to Vorbis, not copy the audio track as-is. I'm quite sure Vorbis is allowed in MKV.

[thany]

Getting another one. Just encoded a video perfectly fine, ow doing another one that has exactly the same parameters everywhere, and this error again. I suspect the video isn't the problem, but what is? I still have no way of know what it is exactly that avidemux is trying to do when this error occurs. So I can't know what it is exactly that doesn't work.

In the mean time I've also upgraded to r9050.

/edit
Exactly the same files with exactly the same settings doesn't throw this error in 2.5, but 2.5 has this problem where audio gets horribly out of sync, so not really an option.

[thany]

Another two. These were video's in a set of videos, all of them encoded at WMV3 1080p 29.97fps and WMA 192Kb.
In all cases I'm encoding them to x264, two-pass 2000Kbps, everything else default. One filter: resize to 720p. Audio is Vorbis, default settings. Container is MKV.

The first two went okay, the second set of two gave this error yet again.
Among these, the filenames are identical, apart from a number in it. I've seen reports that the filename may cause this issue, but obviously that's not it.

This is Avidemux r9050, Windows XP SP3.

Btw, I'm still sort-of fishing for just simply better error messages. I mean, an error message is perfectly alright if there's truly something wrong, but it needs to be descriptive enough to be able to solve whatever it is that's wrong. So even if fixing the problem is going to be too much of a challenge, at least try for descriptive error messages.