Plant SCADA 2023R2 Client machine - cannot access data from server ?

I have just about completed an upgrade to Plant SCADA 2023R2 all clients upgraded (12 in total) with the

Primary Server is on duty with Plant SCADA 2023R2 running , secondary server only just shutdown today (Plant SCADA 2020R2 )to upgrade & finalise the

Upgrade as the last thing to do.

 1 client machine is not establishing comms to the primary server 2023R2 for some reason after I shutdown the secondary server (Plant SCADA 2020R2 - previously when the secondary was on duty this client was fine)

 All settings in Configurator look fine with regard to security roles, mixed mode etc. All other clients ok.

 What can I look for?