When you say all ports are open, does this mean the firewall is inactive or does it mean you opened all required ports? Anyway, please triple check the firewall, to ensure you didn't miss any ports between the Client, vCenter Server and the ESXi hosts. For a list of ports see e.g. VMware KB: Required ports for vCenter Server 5.1.x .
Btw. does the same issue occur with the Windows based vSphere Client?
André