Thanks for details ... so I was in-correct about remotely hosted ... that happens with no info! ;)
Ok, here's some general things ...
The directory that you send the backups to from Moodle should really be a mount point. To debian/Moodle it looks like a local folder, but is really the Windows share. That mount point is typically setup as /mnt/somename. Then one could use a symlink from whatever space the Apache server of Debian can already see/use named 'autobackups' to /mnt/somename. And the FSTAB setup has to be such that apache2 user on Debian can read/write to that mount point.
Example for CentOS: root for apache service is /var/www/
Document root is /var/www/html/ - anything in there is served out by apache ... this is where Moodle code goes. The data directory for Moodle is in /var/www/moodledata
So IF I were attempting to do the same thing with CentOS to a Windows share, I'd make the mount point for the windows share in /mnt/windows. Get that working first. Then in /var/www create a symlink for "autobackups" that points to /mnt/windows
from /var/www/ issue something like: ln -fs /mnt/windows autobackups
To Moodle/apache2 user it looks like the 'autobackups' directory is in it's space ... but really it's on the Windows server. In Moodle config, the path is /var/www/autobackups
In this case would think the permissions need to be wide open to the moodle server. Yep, I know that sounds in-secure and it is somewhat but that directory should not be directly accessible by anyone with a browser ... just like moodledata is not directly accessible. It has to be the 'apache2' user that has access and access comes from ONLY the apache2 server.
Anyone on this forum a Debian person ... feel free to jump in here. CentOS is similar, but all have their quirks ... especially talking to Windows.
'spirit of sharing', Ken