Also having this exact same issue. From some testing I'm wondering if it has to do with the "size" of the VE Pro Instance I'm trying to connect to. For example, I can connect and disconnect without trouble to a VE Pro Instance with a few instances of Kontakt running inside it, but if I try the full 9-Kontakt instance I'd like to be running, this glitch appears.
To reiterate: The local instance appears in the connection list. Selecting it and clicking "connect" causes it to fall off this list. In the VE Pro Server pane, this instance will register as connected. But the plugin within Cubase will still read as "Disconnected."
Cubase 8.0.1
VE Pro 5.4
Mac OS 10.10.3