IPv6 not disabled in mpd.conf w/ IPv6 GUI option set to 'disabled'
When my ISP (Fairpoint) apparently added some IPv6 support, my connectivity broke, even though IPv6 was proactively disabled in my PPPoE config.
set bundle enable ipv6cp
in mpd_wan.conf and running mpd5 by hand fixed the problem.
Attaching logs and a patch that works for me (not tested on actual IPv6 connection).