This report mirrors the issue I submitted to Spacemacs as issue 13866. This
issue
has not received any replies yet. Since then I reproduced the issue with
'emacs -Q'

Link to Spacemacs issue (with diagnostic and traceback info)
- https://github.com/syl20bnr/spacemacs/issues/13866

I run Emacs on MSYS2. On August 20, 2020, I refreshed MSYS2 packages
and Emacs 27.1 was installed.

This problem is generated with 'emacs -Q --debug-init':
- no traceback info generated despite '--debug-init'
- When Emacs comes up, execute command: 'package-list'
- The package list comes up (it has 478 packages)
- The *Error* buffer comes up with the following contents:

Failed to verify signature archive-contents.sig:
No public key for 066DAFCB81E42C40 created at 2020-08-22T05:05:02-0400
using RSA
Command output:
gpg: keyblock resource
'/c/Users/977315/c:/Users/977315/.emacs.d/elpa/gnupg/pubring.kbx': No such
file or directory
gpg: Signature made Sat, Aug 22, 2020  5:05:02 AM EDT
gpg:                using RSA key C433554766D3DDC64221BFAA066DAFCB81E42C40
gpg: Can't check signature: No public key

The offending file path consists of the current directory, followed by
c:/Users/977315/.emacs.d/elpa/...
I verified this by launching emacs from a different directory. That change
would be reflected in the filepath, for example, when launched from the
~/tmp directory:
gpg: keyblock resource
'/c/Users/977315/tmp/c:/Users/977315/.emacs.d/elpa/gnupg/pubring.kbx': No
such file or directory

In GNU Emacs 27.1 (build 1, x86_64-w64-mingw32)
 of 2020-08-11 built on fv-az75
Windowing system distributor 'Microsoft Corp.', version 10.0.17763
System Description: Microsoft Windows 10 Enterprise (v10.0.1809.17763.1339)

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Importing package-keyring.gpg...done
Setting ‘package-selected-packages’ temporarily since "emacs -q" would
overwrite customizations
Package refresh done
error in process filter: package--check-signature-content: Failed to verify
signature: "archive-contents.sig"
error in process filter: Failed to verify signature: "archive-contents.sig"
You can run the command ‘package-list-packages’ with M-x pa-l- RET
error in process filter: Failed to verify signature: "archive-contents.sig"

Configured using:
 'configure --prefix=/mingw64 --build=x86_64-w64-mingw32 --with-modules
 --without-dbus --without-compress-install 'CFLAGS=-march=x86-64
 -mtune=generic -O2 -pipe' CPPFLAGS=-D__USE_MINGW_ANSI_STDIO=1
 LDFLAGS=-pipe'

Configured features:
XPM JPEG TIFF GIF PNG RSVG SOUND NOTIFY W32NOTIFY ACL GNUTLS LIBXML2
HARFBUZZ ZLIB TOOLKIT_SCROLL_BARS MODULES THREADS JSON PDUMPER GMP

Important settings:
  value of $LANG: en_US.UTF-8
  locale-coding-system: cp1252

Major mode: Package Menu

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  buffer-read-only: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug sendmail help-mode mm-archive message
dired dired-loaddefs format-spec rfc822 mml mml-sec epa derived
gnus-util rmail rmail-loaddefs text-property-search time-date mailabbrev
gmm-utils mailheader mm-decode mm-bodies mm-encode mail-utils mule-util
cus-edit cus-start cus-load wid-edit gnutls network-stream url-http
mail-parse rfc2231 rfc2047 rfc2045 mm-util ietf-drums mail-prsvr url-gw
nsm rmc puny url-cache url-auth url url-proxy url-privacy url-expand
url-methods url-history url-cookie url-domsuf url-util mailcap epg
epg-config finder-inf package easymenu browse-url url-handlers url-parse
auth-source cl-seq eieio eieio-core cl-macs eieio-loaddefs
password-cache json subr-x map url-vars seq byte-opt gv bytecomp
byte-compile cconv cl-loaddefs cl-lib tooltip eldoc electric uniquify
ediff-hook vc-hooks lisp-float-type mwheel dos-w32 ls-lisp disp-table
term/w32-win w32-win w32-vars term/common-win tool-bar dnd fontset image
regexp-opt fringe tabulated-list replace newcomment text-mode elisp-mode
lisp-mode prog-mode register page tab-bar menu-bar rfn-eshadow isearch
timer select scroll-bar mouse jit-lock font-lock syntax facemenu
font-core term/tty-colors frame minibuffer cl-generic cham georgian
utf-8-lang misc-lang vietnamese tibetan thai tai-viet lao korean
japanese eucjp-ms cp51932 hebrew greek romanian slovak czech european
ethiopic indian cyrillic chinese composite charscript charprop
case-table epa-hook jka-cmpr-hook help simple abbrev obarray
cl-preloaded nadvice loaddefs button faces cus-face macroexp files
text-properties overlay sha1 md5 base64 format env code-pages mule
custom widget hashtable-print-readable backquote threads w32notify w32
multi-tty make-network-process emacs)

Memory information:
((conses 16 168321 9580)
 (symbols 48 10138 1)
 (strings 32 37790 3595)
 (string-bytes 1 1045947)
 (vectors 16 16682)
 (vector-slots 8 200678 15938)
 (floats 8 35 249)
 (intervals 56 11569 0)
 (buffers 1000 13))