Greetings!

I started to see problems with customizing anything using M-x customize.

When I change something and then press |Apply and Save| I get an error. 
Attached is what I got in the trace when applying and saving 
|debug-on-error| .

Unfortunately I could not reproduce it with -Q because the feature 
|Apply and Save| becomes unavailable. However I could reproduce it 
without any .emacs file.

Below is what |report-emacs-bug| has generated. Thanks for any help.

-Mikhail

In GNU Emacs 27.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.22, 
cairo version 1.17.3)
  of 2020-08-29 built on juergen
Windowing system distributor 'The X.Org Foundation', version 11.0.12010000
System Description: Arch Linux

Recent messages:
Creating customization items ...78%
Loading url-util...done
Creating customization items ...done
Creating customization setup...done
To install your edits, invoke [State] and choose the Set operation
(New file)
Saving file /home/mikpom/.emacs...
Wrote /home/mikpom/.emacs
Entering debugger...
Making completion list... [4 times]

Configured using:
  'configure --prefix=/usr --sysconfdir=/etc --libexecdir=/usr/lib
  --localstatedir=/var --with-x-toolkit=gtk3 --with-xft --with-wide-int
  --with-modules --with-cairo --with-harfbuzz 'CFLAGS=-march=x86-64
  -mtune=generic -O2 -pipe -fno-plt' CPPFLAGS=-D_FORTIFY_SOURCE=2
  LDFLAGS=-Wl,-O1,--sort-common,--as-needed,-z,relro,-z,now'

Configured features:
XPM JPEG TIFF GIF PNG RSVG CAIRO SOUND GPM DBUS GSETTINGS GLIB NOTIFY
INOTIFY ACL GNUTLS LIBXML2 FREETYPE HARFBUZZ M17N_FLT LIBOTF ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11 XDBE XIM MODULES THREADS LIBSYSTEMD JSON
PDUMPER LCMS2 GMP

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

Major mode: Debugger

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 message rmc puny dired dired-loaddefs
format-spec rfc822 mml mml-sec epa derived epg epg-config gnus-util
rmail rmail-loaddefs text-property-search time-date mm-decode mm-bodies
mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader sendmail
rfc2047 rfc2045 ietf-drums mm-util mail-prsvr mail-utils help-fns
radix-tree cl-print debug url-util url-parse auth-source cl-seq eieio
eieio-core cl-macs eieio-loaddefs password-cache url-vars gdb-mi bindat
json subr-x map seq byte-opt gv bytecomp byte-compile cconv gud
easy-mmode comint ansi-color ring edebug backtrace help-mode find-func
apropos cus-edit easymenu cus-start cus-load wid-edit cl-loaddefs cl-lib
tooltip eldoc electric uniquify ediff-hook vc-hooks lisp-float-type
mwheel term/x-win x-win term/common-win x-dnd 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 dbusbind
inotify lcms2 dynamic-setting system-font-setting font-render-setting
cairo move-toolbar gtk x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 16 110836 12650)
  (symbols 48 10470 1)
  (strings 32 28886 2083)
  (string-bytes 1 879796)
  (vectors 16 17077)
  (vector-slots 8 201036 11912)
  (floats 8 39 256)
  (intervals 56 1742 129)
  (buffers 1000 20))