mplayerwin is currently built without IPv6 support.
It would be great if this could be changed: Some (German) ISPs do no longer supply public IPv4 addresses to their customers, but use carrier-grade NAT. Their NAT gateways are often congested, so watching YT videos is no fun using IPv4.
It's enabled in the new build, 37926-2, but I have no way of testing it.
Thanks, Josh! IPv6 support is now enabled, but there's an upstream issue I wasn't aware of: mplayer itself does not have support for IPv6 in connection with Winsock...
The support is probably in ffmpeg, if so it would require prefixing the YouTube URL with ffmpeg://, for example:
ffmpeg://rstp://