Using the new LR4.2, on OS X 10.10.4, it creates (in my example) 443 LRT_x.jpg files, but the final movie ends up being only a 0.13 sec video with what appears to be the 1st 2 or 3 frames. It took about a minute to do so. The progress bar stops at frame 4. The problem is likely user error, but 4.1 did work for me.

Well I just went back to 4.1 and I have the same problem there, so it must be my error.

The jpg's are different sizes, so my bad. (same issue as http://forum.lrtimelapse.com/Thread-rendering-problem) I suspect a problem with my cropping.

Yes that's most likely the problem.