Hi !
After further inspection of the problem, it seems to be related to the web browser I use: Konqueror. The problem is not reproducible with Firefox. I found the following bug report from Ubuntu that seems exactly the same proble, with a patch.
https://bugs.launchpad.net/ubuntu/+source/mt-daapd/+bug/210048
I will try to apply it and see what it leads to. Maybe could you think about integrating this fix if it works ?