I'm not entirely familiar with NAS (Network-attached storage), but it is my understanding that NAS drives are similar to regular external drives inasmuch as they can be access through Window via a drive letter. If that is the case (and there are no credentials needed to access the drive, i.e. username/password) then they should work with FEBE.
Here is the Cloud Services window:

Are you able to use the buttons to browse, test, and explore the drive?
Are you getting any FEBE related errors in the error console (
Tools > Web Developer > Error Console)? Please install the extension for error checking. It is more comprehensive.
Also try running "
Clear FEBE preferences", "
Verify extension directory" and "
Check AddonManager functionality" in
Tools > FEBE > FEBE Options > Advanced. (You will need to re-enter your
FEBE backup destination directory and other backup options after clearing.)
If you are still experiencing problems, try this: Open and clear the Error console. Leave it open and perform a backup. After a few seconds (when the progress meter starts moving again) examine the error console for any
FEBE related error messages (type
febe in the "Search:" box). Copy/paste them here.