V5.1 / Server-Client

The customer's project is on WinCC V5.1 SP1. There are 2 servers and one client.

A few days ago, the customer called, telling us that when he tries to open the project from the client he receives the following message :

"The project on the server was closed.
The local computer is waiting for the project to be reopened on the server.
Click on the cancel button, if you want to open another project."

The project on the server was opened.

On the client computer, we did successfully ping on the servers' IP's and also the names of the servers were appearing on the Windows Explorer -> My Network Places-> Entire Network-> Workgroup.

Is there any idea what might be wrong?
Or what else can we try?

Thanks in advance for your time