In Gpg4win-4.3.0-beta571 with GnuPG 2.4.4-beta132

>echo test | gpg --sign --default-key F8D51DE0EE16E9B57009B8DE458612006D8E6F0D
gpg: Warning: not using 'F8D51DE0EE16E9B57009B8DE458612006D8E6F0D' as default key: Key expired
gpg: all values passed to '--default-key' ignored
gpg: no default secret key: Unusable secret key
gpg: signing failed: Unusable secret key

I'll prioritize this as Wishlist because the options in the "GnuPG System" tab come directly from gpgconf and they are meant to be used by experts (who read man gpg, etc.) and maybe for users who are instructed by an IT administrator to enter some value for some option (so that those users do not need to edit some configuration file).

Yes, only settings from the "GnuPG System" tab are involved

Is this only about options shown on the "GnuPG System" tab?

Just adding this note because a next step I'm also evaluating in my current T5593 configuration status it to temporarily create a new Gpg4win 3.1.16 hybrid configuration by also adding latest GnuPG v2.2.31 to see if all issues I reported here are still present (which is also quite probable).
Also because of T5593 it would just be quite interesting to see if GnuPG v2.2.31 too might experience same T5593 path related error.

Agreed.

yep, the implementation thinks that the default signing key is expired due to metadata contained in the public keyring. The secret key is available to the implementation. So the error mesage No secret key can cause confusion and/or panic if the user thinks they've actually lost their secret key.

You mean the default key is expired?

fwiw, i can reproduce this on debian unstable with gpg version 2.2.17, without a redirected agent -- so the agent redirection isn't relevant here.

3.1.6 will have two ways to install the browser integration non-interactively

Thanks for the report. Indeed I've overlooked this.

We didn't found the time to organize it. There will be a OpenPGP summit this fall organized by Patrick, though

I wonder if there's potential for engaging users remotely? Also, in addition to a workshop, maybe a user interface study of how users learn and interact with the tool? I feel like doing that with people who are relatively light/new users of gpg (like me, currently struggling as I wade thru a mix of docs, some of it outdated) could be beneficial. See also: https://arxiv.org/abs/1510.08555