Access denied on destination

Hi there

I'm trying to set up a backup profile which should backing up data from a server (where SyncBack Pro 5.11.3.0) is running.
The destination folder is on a NAS (Synology DS2121+) which is password protected. In the network settings in the profile is the login and password correctly set and "Test Connection" works.
But when running the profile, I am getting an error in the log for every file: "Failed to copy from XXXX Source : Cannot copy file (5): access denied". I've tried different login settings with full path names, updateted the firmware on the NAS, but I am still getting this errors.
When I try the same profile with a different destination (External Hard Drive), everything works like a charm.

Any ideas?

Thanks