gui login problem

dear all, there is an urgent problem with GUI.

symptom:the server is in Germany, in an another country, our colleagues are using load balancing logon groups to access the server. In one building of the plant, they can login the server, but in an another building, they failed with same accounts and laptops. We set different IP address ranges for the two buildings. In the problematic building, I can ping the message sever. I know there is issue with the network, but what's it?

thank you !