Problem connecting to OpenSSH 6.9+
Support for this has been added to PuTTY (http://tartarus.org/~simon-git/gitweb/? ... 97840245b0), so the best fix would probably be to update the embedded copy of PuTTY to 0.65 (which has some other fairly important fixes too). It's not a new RFC (from 2006), it just took PuTTY a while to add this.
If there are too many local changes and updating is difficult, a quick but bad workaround would be to change the client string from the current "PuTTY_Unidentified_Local_Build" to something that matches the list of putty versions in ssh's compat.c having the SSH_OLD_DHGEX flag.