>>> "UB" == Uwe Brauer <oub@mat.ucm.es> writes:

I bisected the whole afternoon and found the culprits.

* The BUG
First the following commit:

commit 4ce81556b7ad7f7f8c8bb07a7a935114bbe850d0
Author: Lars Ingebrigtsen <larsi@gnus.org>
Date:   Mon Sep 23 18:23:22 2019 +0200

    Make mm-possibly-verify-or-decrypt pass decryption error

    * lisp/gnus/mm-decode.el (mm-possibly-verify-or-decrypt): If
    there's an error, report the error back in a new part (bug#25649).

Introduced a bug, using any compiled emacs between this commit and
commit 84ef1ea8b524f8998fc8674b99cf8069e38dce4f 

results in an emacs, which cannot decrypt smime messages!


* The repair
commit 84ef1ea8b524f8998fc8674b99cf8069e38dce4f (HEAD)
Author: Lars Ingebrigtsen <larsi@gnus.org>
Date:   Thu Sep 26 16:24:29 2019 +0200

     Make it possible to view S/MIME verified emails

     * lisp/gnus/mm-decode.el (mm-possibly-verify-or-decrypt): When
     dissecting the result, we need a header (bug#18393).

     * lisp/gnus/mm-view.el (mm-view-pkcs7-verify): Insert the verified
     string.

     * lisp/gnus/smime.el (smime-verify-region): Return the verified
     string.

With this commit, smime message can be decrypted, but they are not
decoded well, at least not for my setting, and my setting worked well
before commit 4ce81556b7ad7f7f8c8bb07a7a935114bbe850d0

So Lars, could you please try to repair this?