Welcome to Nextcloud
Maybe you dont know what i mean.
I have a synology nas where my nextcloud is installed as a service.
I now want to connect my shared folders (created on my nas) to nextcloud, so i can still access the files via network access on my windows client.
Via USB Copy i want to export and backup my files to an external HDD which is connected via usb to my synology.
In the external Storage app i now want to connect this shared folder, but no matter what method i choose i always get this red error sign. 
Okay i was able to do it.
I followed this tutorial and used WebDAV to do it.
are you able to connect NAS to ftp?
Thanks a lot! I have been fighting with this since yesterday, and now I come by your comment. And it WORKS!