> From: Gábor <hajovonta@gmail.com>
> Date: Sun, 11 Oct 2020 17:09:27 +0200
> Cc: Lars Ingebrigtsen <larsi@gnus.org>, 43888@debbugs.gnu.org
>
> I have the with-editor package, but: a) it's up-to-date b) removing it from the configuration did not solve the
> problem, c) running emacsclient from outside of Emacs gives the same error. :(

But since with "emacs -Q" it does work, it is still some of your
customizations that are the prime suspect.