Is the avast NetAgent service running on the clients?

Yes, the NetAgent service is started.

Also, what is the value of the ServerAddress= line in \data\avast4.ini (on the clients)? Does it have the correct server name in it?

Yes, the ServerAddress= line (on the clients) has the correct server name (in the form of myserver.mydomain.com).

Thanks for the quick reply btw ;D