I've had frequent crashes in my 10.4 OneSwarm setup for a couple of months, which annoyed me to the point that I even reinstalled OSX and ran the hardware test, more info here:
http://forum.oneswarm.org/topic/1411
Now, while this isn't certain to be the cause of these problems, I do have a problem indeed, and a reproducible one. My swarm preview generation is completely broken, and thanks to the FFMPEG debug output feature, I think I've figured out why!
Any added video file ends up stalled, and the FFMPEG output ends with "Must supply at least one output file".
The input appears to be OK, such as '/Users/user/folder/movie.avi', no spaces as you can see and it doesn't appear to work much better with spaces either ;-) Worth noting is that the other argument (not shown in the output) should be something including "Application Support", and that might be where things go wrong, with FFMPEG being famous for its hypersensitivity to spaces?
I tried downgrading to the FFMPEG binary included with FFMPEGx (April 15 2006) but got the very same result. Previews have worked onetime (less than 1 year ago) with this very same setup, since then however there have been 1S updates as well as minor java and maybe even OSX?
System:
iMac G5
OSX 10.4.11 (fresh install)
JVM 1.5.0_16
Running OneSwarm 0.6.9.0 as regular user (non-admin)