Dear all,
The problem solved when we implemented the following workaround.
We used the ERA console so as to direct the clients to retrieve an update from the internet. As soon as they updated their signatures, we re-configured them in order to synchronized with our internal mirror update server.
It seems that the client retrieve a signature that was not available for the mirror update server even though the server had the latest virus signature version.
I know, it does not make sense but it is working.
thanks.