LG G3 (d855) Lollipop: cannot set custom ringtone
Thanks to the useful suggestion of a forum post I managed to fix by


copy the ringtone in the /Music~ folder (instead of/Ringtones`)
start the bundled music player, play the song I want for ringtone, then in the menu select Use as ringtone


This way I could finally change the ringtone with a custom one!