Vim 9 problem under Cygwin64

I'm hoping someone can help with a problem. I've had cygwin64 installed for at least a couple of years, with Vim v8 running just fine. I also have Windows Vim running; this is all under Win 10.

 I've just updated the cygwin64 installation and vim is now v9.0. When I start the cygwin vim I get these lines:

===========================

Error detected while processing /etc/vimrc:

line   90:

E484: Can't open file C:UsersOwner_vim/vim90/syntax/syntax.vim

Error detected while processing /home/Owner/.vimrc:

line    6:

E484: Can't open file C:UsersOwner_vim/vim90/syntax/syntax.vim

Press ENTER or type command to continue

===========================

Pressing ENTER allows me to continue, editing files, etc. but not surprisingly with no syntax highlighting.

:echo $VIMRUNTIME = C:\Users\Owner\_vim/vim90
Note the mixed up forward and backward slashes in that path. Note also the lack of slashes in the paths reported above as E484. 

 It seems clear that there is some sort of clash between Unix and Windows path conventions going on but I haven't been able to figure out how to fix it.

The file syntax.vim does exist and is in the right location (I think):

$ ls -l /usr/share/vim/vim90/syntax/syntax.vim

-rw-r--r-- 1 Owner Administrators 1.4K Dec 20 01:57 /usr/share/vim/vim90/syntax/syntax.vim

vim --version includes these lines:

VIM - Vi IMproved 9.0 (2022 Jun 28, compiled Dec 20 2023 06:57:02)
Included patches: 1-2155

   system vimrc file: "/etc/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"

 

One more item:

:scriptnames

  1: /etc/vimrc

  2: ~/.vimrc

  3: ~/.vim/keymap/insert_only_capslock.vim

  4: ~/.vim/bufferlist.vim

  5: ~/.vim/plugin/matchit.vim

Thanks for any help!

John Cordes