Running Bitcoin-qt on an IPv6-only Windows server 2008 R2. Bitcoin thinks there is IPv4 and binds to 0.0.0.0:8333 and tries to connect to IPv4 nodes and gets stuck. Restarting with addnode to connect to a known ipv6 node works..
debug.log:
Bound to [::]:8333 Bound to 0.0.0.0:8333