This problem is the subject of several previous posts http://www.objectiflune.com/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=27;t=000283
http://www.objectiflune.com/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=27;t=000205

It occurs randomly in V4 Watch, so you should almost certainly get a clean file if you rerun the job.

In the current version (V5.3) the problem is fixed.