But this problem remains for several versions for some time. I tried to find out the source of this "new option" in the communication, but I could not find anything about "GPG Agent" in the source code of openssh.

Sorry for the late answer, but I have been busy. Actually this happened against several ssh versions, for some time now.

This is a high prio error, I guess, because it breaks a very useable part of gnupg, that is really hard to maintain. If it is not stable to sign keys with the gpg-agent, it is very hard to use that. Many might switch back to the ssh-agent.

Please check if this patch works for you and please check where this flag actually comes from and what it does say!