Cannot get this version od 2.6 to do any 2 pass MP4 conversion - programme errors.
Have had to go back to 2.56-1
Is there a another version of 2.6 due out?
This is very vague what you're telling us? Seems like I just done that yesterday? see: Encoding with x264 using "video size (two pass)" not working (http://www.avidemux.org/smf/index.php?topic=11400.msg59679#msg59679)
You need to elaborate more on this: source video, your steps in avidemux 2.6 r8194, OS, 32/64-bit, ... ?