Quote from IRC:
hey, i've some problems with my smartcard since quite some time. i'm not sure whether it's openssh related or gnupg. it's a openpgpcard v2.0 and i have to workaround ssh logins by using "SSH_AUTH_SOCK=0 ssh ...". .gnupg/gpg-agent.conf -

Yeah but it seems to be the same issue / reason. I wasn't aware that PKISSH is something else. I thought it was an extension/protocol or something

I was and I am using OpenSSH on both sides, client and server.

Well, I have no idea about the technical background to be honest but without this patch it doesn't work at all for me, unless I stop using the agent or workaround it by using SSH_AUTH_SOCK=0. With this patch, I can use the agent again. I don't know how many others are affected by this but it made it usable again, which wasn't the case for months already.

I am affected by the same bug and the patch seems to work for me. Login via gpg-agent with ssh support is possible again, which wasn't before, since some openssh and/or gnupg update. Not sure.