Muxing stops with an error message saying that it would have to split the file but a parameter %1 is missing. I have not set the split option.

It happens with one specific video at approximately the same spot when MPEG Streamclip finds a single Timecode Break.

TOAST and Moviegate mux the video files just fine. However, I need two Audio Tracks on the authored DVD, so ffmpegX is the only application that offers the possibility to mux two audio tracks.

I was checking mplex options:
--ignore-seqend-markers|-M
Don't switch to a new output file if a sequence end marker
is encountered ithe input video.

Is the option used by ffmpegX? I identified the timecode break most likely to be a layer break. (MTR shows that in mode menu)

I think I give mplex a manual try (that will be an adventure)....