whats the solution to this "avconvert failed to create an export session"

It would appear that avconvert and many other automation solutions are broken under 10.8 (Mountain Lion). For example, the Encode Media service described here fails silently on MacOS X 10.8.

I suspect that this is a sandbox issue because avconvert is right where it belongs in /usr/bin/

In MacOS X 10.8, there is a new feature that Apple calls "Gatekeeper." It's goal is to provide greater security against code being executed that you did not intend. This is the essence of what trojans, viruses and social engineering try to do: execute malicious code on your system. One way to militate against that kind of attack is to "sandbox" everything so that code in one area cannot execute in another area.

Did you find a fix, I have the same issue

This may seem like brute force/overkill but here's what worked for me:

1) Boot into Recovery disk. Hold down Option key while booting and all bootable volumes will be shown. Choose Recovery.

2) From the options presented, choose Re-install MacOS X. Be aware that this takes a loooong time. I let it run overnight.

Optional: Run Disk Utility > Repair dis and repair permissins first. Probably not necessary but I wanted to be very cautious.

Did anyone find the solution to this error? I am facing this problem with Maverick, all latest apps/updates, but only on some of my home video files, mainly multiplexed MPEG-1/2.