Authentication Failed or Access Denied

[hsmal1]

I have installed Veyon on Windows Server 2016 and completed the configuration to use AD. In Configurator, when I search a user or a computer I get the info back correctly.
On a local PC I have installed Veyon and loaded the configuration from the Configurator on the Server. I see the remote PCs OK and, in one group of four, one is red and the other three are grey. If I hover over the grey ones it tells me they are off line or switched off. If I hover over the red one it tells me Authentication Failed or Access Denied.

Can someone please tell me the cause of the authentication/access problem? Thanks.