When I installed my new OMV 2 days ago, during the install I opted for a blank root password, so root login is disabled and a sudo user a/c was created. I then spent a few hours trying to figure out why I could not connect to any smb shares. The penny almost dropped when I was successful with the admin user.
It looks like the new user's password is not sync'ed with Samba. Once I updated the password in the GUI everything magically worked.
Regards,
Peter.