Cannot write to backup file - vim

Question

Since I have upgraded to Catalina, I cannot edit my dotfiles but get a message saying "Cannot write to backup file...".
I have checked the permissions on the file and my user account is the owner and has read and write permissions. If I edit the files with BBedit, I can save the files so the issue just seems to be editing within neovim.
I can also edit files using neovim in other directories but not within my dotfile directory.
Additional information added:
I have noticed that the files that I can't edit have the following permission structure:
.rwxrwxrwx# 1.4k pdd 30 Aug 2017 plugin_manager.vim
I have now removed the # attribute using xattr but that has not made any difference.
Additional Information Update
I have narrowed it down to something in my vimrc file. If I start neovim without my vimrc, I can save files in my vimrc directory.
I have the following code in my vimrc relating to backup files:
if exists('$SUDO_USER')
set nobackup " don't create root-owned files
set nowritebackup " don't create root-owned files
else
set backupdir=~/local/.config/nvim/tmp/backup
set backupdir+=~/.config/nvim/tmp/backup " keep backup files out of the way
set backupdir+=.
endif
If I comment these lines out, I can then save files in my vimrc directory. I am a little confused as I have had these in my vimrc for some time and have not had a problem.
I am probably missing something simple. Does anyone have any suggestions?

Answer 1

The original backup I used for vim was found here: https://vim.fandom.com/wiki/Incremental_backup_in_central_backup_directory created July 22, 2005 author Sylvain Lafleur
It doesn't work in neovim, but I was able to get it working:
I'm using KDE Neon. The original looks like it might work on windows, but I have not tested it.
A few things to note:
You will need to manually edit the g:backupdir and
g:this_root_backkup_dir (This may be reduced, but I didn't mess with it.)
I was unable to make $HOME work, so use /home/username on linux.
This is not my actual directory structure, but should suffice as an
example, in that /home/neon/vim existed before I used this code.
This is very redundant backup. Every time the file is saved it will
create a backup of the filename with date/time stamp.
In the .vimrc put your vim specific code in if !has('nvim')
and endif blocks. I put the following code in my init.vim file.
set backup
set backupcopy=yes
function Write_backup()
let g:backupdir='/home/neon/vim/vim_backups'
let &backupdir=g:backupdir
let g:backupext = strftime("_%Y-%m-%d_%H-%M-%S")
let &backupext=g:backupext
let g:this_root_backup_dir = '/home/neon/vim/vim_backups'
let g:this_dir = substitute(expand("%:p:h")," ","_","g")
let g:this_filename = substitute(expand("%")," ","_","g")
let g:this_drive = strpart(g:this_dir, 0, 1)
let g:this_backup_dir_drive = g:this_root_backup_dir . g:this_drive
let g:this_backup_dir = g:this_backup_dir_drive . strpart(g:this_dir, 1)
"--make DRIVE directory if it doesn't exist
if !filewritable(g:this_backup_dir_drive)
silent! execute expand('!mkdir -p ' . g:this_backup_dir_drive)
endif
"--make directory under DRIVE if it doesn't exist
if !filewritable(g:this_backup_dir)
silent! execute expand('!mkdir -p ' . g:this_backup_dir)
endif
"--set new backup dir
let g:backupdir = g:this_backup_dir
let &backupdir=g:backupdir
endfunction
call Write_backup()
inoremap <ESC> <ESC>:call Write_backup()<CR><C-l>
" note the <C-l> clears the output so remove if testing with echo