load-balanced server logs fail

[Leif]

Urchin 6.3: When I process two large (~4GB) logs from load balanced servers, the first completes and the second processes very slowly and then just stops after about 15% completion.  The Profile Task History shows a status of "Error". I've looked in all the logs for any explanation and can't find anything unusual. In fact, I don't see any log that even reports there was an error. Can anyone suggest how to debug this problem?