Some PCs can't connect to share via domain\user but via user@domain

Hey

Not sure where else to ask really so I hope I can get some Info about that problem I am facing.

Out of over 100 clients, we have a group of 8 PCs that cannot connect to a share that has to be reachable in order for our backup to work. If you try to manually connect to the share via domain\\user when the login prompt appears, it says that the Username and Password is wrong even though it absolutley is correct as we use it that way for over 100 machines.

If we use the alternative syntax of Username@domain it works.

Though I don't know how I can tell our backup software how to use that alternative way, I simply need domain\\Username to work again.

It did work around 3 weeks ago until it suddenly stopped working and I got no clue why this happens on those 3 machines.

All clients in the backup are not domain joined but trying to reach a server that is inside the domain. Never was a problem before.