Ok, I quit out of VE Pro, saving the project with a GR4 FX in each. I get the message 'another instance is running', + 'will not save to database', which I have seen with Kore 2, but it never caused me a problem I can recall. it doesn't cause any hang here...
Both my Cubase and VE Pro projects, both servers connected to Cubase are stable with a GR4 FX plugged in to each server, and I have saved the metaframes. I loaded a cab to my GR4 in the 32 bit server it is plugged into and saved the thing in Guitar Rig. For me this is a quirk and not a problem. Technically, there *is* another instance running. I don't know about the problem of the database it indicates in either Kore or GR4. I'm not sure there is any.
From your description, it appears your PT and VE Pro are coupled... I think the difference, you having hangs where I don't, will owe to the coupling of the two. NB: I didn't quit Cubase, I believe that's irrelevant, as mine is a decoupled situation.
Also, this seems like a problem where if you have dynamic IP that would complicate things if not be a cause.