I had this exact same problem, but when exporting using the Vimeo h264 tweaked preset. The tweak I made was not to export the audio. It then kept making .m4v files, not the .mp4 files and I couldn't play them. I too had no problems with CS5.5

I got around it by simply not unchecking the export audio. It then produced the correct mp4 file and I could play it.

I had the exact same problem & resolution for trying to produce H.264 Youtube HD 720p 25 video files.

Yes Keith, Mac OS X 10.8, and no, the m4v output did not play back at all (I tried Quicktime 7 & 8 and VLC), and the 'file' command line utility did not recognize the contents. A superficial hexdump check shows that the file does not have the typical header for 'proper' M4V video files (signature 00 00 00 14 66 74 79 70), so it really seems to be the bare video stream.

I will have to try keeping the Export Audio box checked.  I am having the same .m4v problem when trying to save as mp4, and my videos have no audio (screen capture tutorials) so I chose not to export the audio.  I will see if keeping the box checked works.

I am using Windows 7 64-bit, CS6 Master Collection Student Teacher;  I just downloaded the full version (paid for) a couple of days ago, so apparently, this is still a bug.

I had posted this same type of question earlier today, now I found this forum.  I don't know if the problem was there before the update because I just bought my Adobe products the other day (November 10, 2012) and installed the updates prior to even opening any single one of the Adobe programs.  Oddly enough, Handbrake does a fine job, so I am using that until I figure out the problem with AME.

UPDATE: 

YES!  Keeping that Export Audio box checked resolved the problem.  What I did was (since there was no audio in the original) just take the audio settings down to the lowest possible quality so as to save on file size output.

Thanks for the pointer!

To reproduce the bug you should import footage without audio. Then you export it without audio. There is 100% chance that you get corrupted m4v file instead of playable mp4.

Just experienced this issue for first time today---just got cs6--when I exported a prppro sequence without audio to h.264 legacy format then it gave me a good .mp4 file that was playable-but the regular h.264 gave me an unplayable (in vlc, playable in windows media player) .m4v file

quick question

what's the difference between the h.264 and the legacy h.264--I guess the mainconcept codec is not used for the new h.264?--Is that it--is there some reason we should not use the legacy format anymore?--i just assume it's more compatible than the "new" h.264 in ame and the legacy still works well for me-so I'd like to continue using it until adobe works out the kinks on the newer version--any thoughts?

Tony

Same problem here.

This is still borked.

I'm trying to make an mp4 file to use for sound design and it won't let me.

Help me, Adobe Tech Support. You are my only hope!

Do you installed the latest update? I installed it and it fixed my export problems.