Reluctantly I reduced the resolution from 1080 to 720 after I got the message "no frames returned". Now it renders quite happily, but towards the very end I get the Message: "Application failed to return a video frame." What is behind that? Anyone? (I am running CS3 on an 12gb i7 intel iMac, Snowleopard)
Beats me, without details.
Often, failure to "Return a Frame," is due to a gap in the Video. If you are getting a %, or a Frame Count, that is where I would begin looking for gaps. Even a single Frame can cause this.
When you find the gap (could be others, so check the entire Timeline), how you close it will depend on whether you have Handles on either Clip, and what you wish to have happen at that point.
If you are 100% positive that no gaps exist, you might want to take a look at this ARTICLE for the types of info, that would be so useful in trying to help.