.. er ... dunno
I'm guessing you've checked all this but:
Is there any 'missing media' shown by hash lines on the clip in the timeline?
Is there anything different to your previous projects - maybe bitrates etc?
Have you used something different to capture your images?
The next thing I would do to try and isolate the problem is:
Duplicate the project
Working with the duplicate
Delete the first clip
Try exporting the project
If it fails, it's not that clip so use the 'back' button to re add it
Delete the next clip