"Error -2048: Couldn't open the file (blah blah blah).mp4 because it is not a file that QuickTime understands."

All footage is 1280x720 30p AVCHD. Project runs approximately 40min. in length. No crazy grading, transitions, or pictures. It seems like any time I have a rendered project of any significant length and I choose MainConcept AVC .mp4 (@14Mbps), it doesn't seem to want to play in anything. Quicktime, VLC, etc.

I'm sure I could try a different format, but I want to use AVC/mp4. Am I doing something wrong? I render this way all the time on smaller projects, but anything larger gives me trouble.

Thanks for any help.