Are you using CloudSync's encryption feature? That's meant more for using Dropbox (or whatever) as the mirror and giving it encrypted files which your NAS nodes

Are you using CloudSync's encryption feature? That's meant more for using Dropbox (or whatever) as the mirror and giving it encrypted files which your NAS nodes will transparently decrypt, like Synology's version of Boxcryptor. To back up data from Dropbox, I'd create an encrypted Shared Folder on the NAS, turn off CloudSync encryption, and sync into the folder with the sync direction set to pull-only. That gives you a mirror with local file system-level encryption (using ecryptfs; buyer beware) and keeps Dropbox as your source of truth. There may be a different way to do it with DSM 7, but I've only tried this on 6.