Can't make backup file (add ! to override)

If anyone else is having this issue you might have configured your backup directory in your .vimrc.after or .gvimrc.after file. The latest version of Janus sets your backup directory to .vim/_backup but you can easily see what it's been set to and if you have possibly overridden it by running:
:verbose set backupdir?
This will print out something like the following:
backupdir=~/.vim/backup
        Last set from ~/.vimrc.after


Either remove that line from your .after file and leave it up to Janus or make sure that VIM has write access to that directory.