I do not own either, but what about trying with the Synology IP address instead of the name? i.e. smb://192.168.1.xxx/RoonBackups. If ROCK for some reason can not resolve name-to-ip you’d get errors like this.
ROCK and your Synology is in the same IP subnet? Does the sharing on Synology contain any ‘allow’ rules or anything that can prevent ROCK from connecting?
Let us ping @support here to make sure they see your request for help.
Also this thread deals with the same issue
Could you try connecting to the share using the CIFS protocol instead of SMB? To do so, just replace the “smb://” with “cifs://”. We are actively investigating a few bugs relating to SMB and NAS usage on Linux cores and hope to have some more fixes out soon.