Last year october we updated our CentOS 7 server.
Since then we cannot access it's Samba shares outside it's own local subnet. Before the update we were able to connect to those shares from other subnets as well.
When connecting from the local subnet we can reach the shares without a problem.

When I connect from a machine outside the subnet of the CentOS server we are prompted for a username and password. I enter the same credentials as I would enter from the machine within it's subnet. The prompt keeps coming back, asking for credentials.

As soon as I connect to the share from outside it's subnet, even before trying to log on, this is what pops up in the Samba logging:
Failed to find a Unix account for <local username of the remote machine>

The stated local username is not what I enter when I'm asked for credentials.

As I am being asked for credentials and because the Samba logging says something about my useraccount I think this has nothing to do with a firewall.

We tried updating Samba again, restarted the CentOS server and tried to log on from several devices outside the subnet with Windows 7 & Windows 10. Nothing has worked so far.

Anyone seen this before?