Many job fails

[mase]

Hi!
I am using the latest git build of 2.6 on Debian Jessie.
I prepared a lot of jobs to encode 1080i TS VDR recordings to XVID.
But about half of the jobs fail. It is not reproducable, because when
I reset the failed jobs to "ready", about half of them succeed. I have to
repeat this, until all jobs succeeded. I made no changes to the
jobs. I use the QT4 version to prepare and run the jobs.
How can I find out, where the problem is?