Suggestions for backup of a qnap nas

hi to all. I want to know some best practice to backup a qnap ts212p. this is the situation:
server proxmox with a virtualized kvm debian and last urbackup server. I backup everything in my office network with urbackup client, linux windows mac everything is perfect. but obviously I cannot install the client on a qnap nas, so I mounted the nas share on urbackup server and run a client on the server for backup the mounted share. this works but if someone in the lan have a file opened(lile an excel word etc) I will receive some. warnings in the logs like these

2017-11-11 11:04:05(info): backup: Started loading files…
2017-11-11 11:04:05(info): Referencing snapshot on “backup” for path “anektux3” failed: FAILED
2017-11-11 11:46:38(warning): Error getting complete file “3CfSKxEbAhH16CCigEIs|anektux3/pubblica/DATI 2017/EVENTI DIVERTENTI.xls” from backup

any suggestions?? is this the correct way?
many thanks

You can probably configure the NAS (samba) such that it does not lock files, or mount the share via NFS or something.

Really the best solution would be to properly integrate it with QNAP and snapshots there, but somebody will have to create a QNAP app for that.