When connecting to the forwarded port of the server using IPv6, accept fails in channels.c:channel_post_port_listener() (invalid pointer) on windows. The problem disappears, if struct sockaddr is replaced with struct sockaddr_storage.
All the address arguments are sockaddr_storage now. This will be in the forthcoming openssh-5.1 release. If you would like to test this release, please see the email at http://marc.info/?l=openssh-unix-dev&m=121538716918994&w=2 Thanks for the report.
Mass update RESOLVED->CLOSED after release of openssh-5.1