I was busy configuring external storage support for some of my users. During this I found that when I wan’t to authenticatie against a domain joined windows smb server and using the logged in credentials (with use of ldap/ad integration) it would not send the additional @domain part of the username. This obvisouly results in an authenication failure.
When using the samaccountname it is working correctly because it does not use the @domain part. When using UPN as sign-in the problem occurs.