Quoting David Bremner <david@tethera.net>:

Jeff Elliott <jeff@fedge.ca> writes:

[Wed Sep 22 15:49:36 2021]
A caller requested output format version 5, but the installed notmuch
CLI only supports up to format version 4.  You may need to upgrade your
notmuch CLI.

When I try to upgrade my notmuch from the Solus package manager, I get
that it is already updated.

Yep, this is exactly the problem mentioned in the README on melpa (which
I'm guessing is where you installed notmuch from). You need to use a
version of the notmuch CLI that is as new as as your version of
notmuch-emacs, or an older version of notmuch-emacs. Normally I would
recommend using the notmuch-emacs package from your distro, but I
couldn't figure out if it exists in solus.

If you have the appropriate version of the source, then "make elpa" will
create package you can install in emacs

   M-x package-install-file <return>
   notmuch-0.31.4.tar <return>
should work. If you're stuck, I can make such an elpa package for you.

Thanks. I guess I'm still stuck.  I downloaded version 31.4 from:
https://notmuchmail.org/releases/

But when I select "notmuch-0.31.4.tar" for the package-install-file, I get an error:
 Wrong type argument: stringp, nil

Would it not make sense to upgrade notmuch to 0.33.1 ?  Which I guess I would
have to do from source, which I haven't done before...