Serviio

I see that in many case metadata are created in the mediafolders, both for video and audio ( metathumb, xml files ).
How can I avoid this ?

The audio metadata setting : 'look for local album art image' should not create new files, or should it ?

Serviio doesn't create any 'files'. When it does its metadata thing, it reads the file, and extracts the metadata that way. If it doesn't find any, it will use the naming conventions to try and figure out the media being presented. As far as the check box, that will override any covers that it pulls down. For example, if you have a Directors Cut or something, you can use that cover image instead of the "normal" cover that came with the media.

I'm guessing what's happened here is that at some time you've accessed your files with your media player via a network share, rather than via Serviio, and that the media player is set up to automatically save metadata. I've had similar files appearing after doing that with my WD TV Live.