Conversion / Optimization stops after each title but cue is not empty

I am converting / optimizing the entire library with the “TV” profile but after each title the conversion stops. All titles have status “waiting”. I have to pause and release all items (by the pause button) and then the next movie is going to be converted. Then it stops after that movie again, and so on.
I deleted the cue, started all over again, rebooted the system, and so on.
This effect only happens with that particular library (>100 movies). With smaller libraries it worked quite well in the past. I don’t know if this is the reason but at least a connection.

I run the server Version 0.9.16.4 on a QNAP TS-453 Pro with Intel Celeron and 8 GB RAM.

no help here?

The problem still exists. What can I do? Any idea?