As I am new to buildroot I am not fully sure whether this is a bug or a misunderstanding on my side... The music player daemon config in package mpd makes use of files in folder "/var/lib/mpd". However, this folder is not created by the makefile. A simple "mkdir -p /var/lib/mpd" in mpd.mk may do. Maybe also adding the subfolder "music", "playlists".
Fixed by https://git.buildroot.org/buildroot/commit/?id=d0031c5621e302c659bec13dfcc62b55ac288462, thanks