Hello, I've been working in this stuff, and from several days making test, I've found 2 ways to solve this problem.
I think the problem happens when the Synology is a member of the domain (not sure at all).
I was using a local user of the Synology NAS to make the backups, it works fine in SmeServer9.2 but in SmeServer10 appears the error "mount error (13): Permission denied" (doesn't mather which version of the Synology DSM used (6.2 or 7)).
The only thing that I've found to be changed is the version of CIFS in SmeServer.
But if you use a domain user (with proper access permissions in the backup shared folder) it would work fine.
Also find another solution, more than solution its a trick:
For use a local Synology account, you must specify the CIFS option: domain=.
It will say to the Synology that you want to use a local user.
For example, the command will be like this (SmeServer):
/bin/mount -t cifs "//$host/$share" $mountdir -o credentials=/etc/dar/CIFScredentials,domain=.,nounix
I've thought about change this part of the code to add the domain option, but maybe in the next SmeServer update it will come back to defaults.
Also, I try to change the credentials (/etc/dar/CIFScredentials) and add a line with "domain=." but when you activate the backup via server-manager this file it's overwritten and disappear the domain option.
The last one is to specify the domain option in the username value, like this:
User Name: [local_user_name],domain=.
You can pass domain option using this field, because the credentials file can contain user, pass and domain.
But it's a trick that now can work but maybe not in newer versions of SmeServer.
So, resume:
When Synology its a member of the domain you don't need to specify the domain if the user is also a member.
If the user is not a member and its a local user you have to use the option domain=.