Do you have this?
2020-03-31_09-30-31.png
Anyway, the batch processor is always decoding files and reencode them. Hence this is not good to process mp3 files. You should rather use wav files and convert them to mp3, with the proper metadata.
Thanks PG, appreciate your fast support! That did the trick with the image file 
In the batch processor, the files need to be in the right order. Then, you must set processing to a single core in the batch options, else files are processed in parallel, hence in no order.
There are several issues here.
If you mean the execution tab and usage of processor cores, yes. 1 Task is selected here but it doesn´t work.
All files that I process are called track-name, track-nr.1
The problem with setting the correct order is that Wavelab auto-sorts the files in a wrong way and I can´t do anything about it.
E.g. a track with the name track 10 is always placed BEFORE track 2. It looks like this:
Track 1
Track 10
Track 11
Track 12
Track 13
Track 14
Track 15
Track 16
Track 17
Track 18
Track 19
Track 2
Track 20
Track 21
Track 22
Track 23
Track 24
Track 25
Track 26
Track 27
Track 28
Track 29
Track 30
Track 3
etc…
There is no option in the process batch window to sort the files manually by name.
But I´m not sure if the proper order in this window would solve my problem.
It seems that a Name Scheme option, specifically for MetaData is missing in Wavelab.