The last status I heard on FTP was this from two days ago:
Possibly this one, which was a bug found and fixed after Beta release.
Fixed problem with FTP backend not unescaping special characters
If this is your situation, you can try renaming the folder to match Duplicati use.
Using latest Canary test release is more drastic. ETA of next Beta is unknown.
Since NAS is probably Linux, a symbolic link might make it work in either case.