This post comes up in Google the highest for anything resembling this problem, and now that ffmpeg is falling by the wayside in Debian 8 and gmus hasn't yet updated to gstreamer1.0, I think it's important to have an actual fix for OP's question.
The fix for mplayer complaining about an unreadable input.conf after one song is simply to create an empty ~/.mplayer/input.conf. Press 'play' and gmusicbrowser will happily progress to the next song in the playlist. You don't even have to restart GMB. This doesn't account for broken input.conf's, just missing ones (which mplayer will complain as unreadable).
The fix for mplayer complaining about an unreadable input.conf after one song is simply to create an empty ~/.mplayer/input.conf. Press 'play' and gmusicbrowser will happily progress to the next song in the playlist. You don't even have to restart GMB. This doesn't account for broken input.conf's, just missing ones (which mplayer will complain as unreadable).