Vim shows the following long error message when I
open a file that has a dot-ini (.ini) file extension:

Error detected while processing
/usr/share/vim/vimrc[25]../usr/share/vim/vim90/syntax/syntax.vim[44]..BufRead
Autocommands for "*~"..BufRead Autocommands for "*.ini"..FileType
Autocommands for "*"..Syntax Autocommands for "*"..function
<SNR>3_SynSet[25]..script /usr/share/vim/vim90/syntax/dosini.vim: line 28:
E10: \ should be followed by /, ? or &

Implementing that suggestion at the end of the message had no effect. (Or,
rather, it made the error message even longer and more incrutable.)

--> vim --version

VIM - Vi IMproved 9.0 (2022 Jun 28, compiled Aug 15 2023 20:47:29)
Included patches: 1-1713
Modified by team+vim@tracker.debian.org
Compiled by team+vim@tracker.debian.org
Huge version without GUI. Features included (+) or not (-):
+acl +file_in_path +mouse_urxvt -tag_any_white
+arabic +find_in_path +mouse_xterm -tcl
+autocmd +float +multi_byte +termguicolors
+autochdir +folding +multi_lang +terminal
-autoservername -footer -mzscheme +terminfo
-balloon_eval +fork() +netbeans_intg +termresponse
+balloon_eval_term +gettext +num64 +textobjects
-browse -hangul_input +packages +textprop
++builtin_terms +iconv +path_extra +timers
+byte_offset +insert_expand -perl +title
+channel +ipv6 +persistent_undo -toolbar
+cindent +job +popupwin +user_commands
-clientserver +jumplist +postscript +vartabs
-clipboard +keymap +printer +vertsplit
+cmdline_compl +lambda +profile +vim9script
+cmdline_hist +langmap -python +viminfo
+cmdline_info +libcall +python3 +virtualedit
+comments +linebreak +quickfix +visual
+conceal +lispindent +reltime +visualextra
+cryptv +listcmds +rightleft +vreplace
+cscope +localmap -ruby +wildignore
+cursorbind -lua +scrollbind +wildmenu
+cursorshape +menu +signs +windows
+dialog_con +mksession +smartindent +writebackup
+diff +modify_fname +sodium -X11
+digraphs +mouse -sound -xfontset
-dnd -mouseshape +spell -xim
-ebcdic +mouse_dec +startuptime -xpm
+emacs_tags +mouse_gpm +statusline -xsmp
+eval -mouse_jsbterm -sun_workshop -xterm_clipboard
+ex_extra +mouse_netterm +syntax -xterm_save
+extra_search +mouse_sgr +tag_binary
-farsi -mouse_sysmouse -tag_old_static
system vimrc file: "$VIM/vimrc"
user vimrc file: "$HOME/.vimrc"
2nd user vimrc file: "~/.vim/vimrc"
user exrc file: "$HOME/.exrc"
defaults file: "$VIMRUNTIME/defaults.vim"
fall-back for $VIM: "/usr/share/vim"

Compilation: gcc -c -I. -Iproto -DHAVE_CONFIG_H -Wdate-time -g -O2
-ffile-prefix-map=/build/vim-9Zy3tT/vim-9.0.0749+really.v9.0.1713=.
-flto=auto -ffat-lto-objects -flto=auto -ffat-lto-objects
-fstack-protector-strong -Wformat -Werror=format-security -D_REENTRANT
-U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=1 Linking: gcc -Wl,-Bsymbolic-functions
-flto=auto -

:set runtimepath? shows:

runtimepath=~/.vim,/var/lib/vim/addons,/etc/vim,/usr/share/vim/vimfiles,
/usr/share/vim/vim90/,/usr/share/vim/vimfiles/after,/etc/vim/after,
/var/lib/vim/addons/after,~/.vim/after

I have vim-plug, the vim plugin manager, in ~/.vim/autoload/plug.vim but no
plugins currently installed,

How can I get Vim to open .ini files without showing error message(s)?

Thank you for reading.