ubuntu 16.04 will not join session

I can see the Ubuntu 16.04 PC I want to control remotely is registered correctly with Connectwise, and is waiting for me to join it's session from the rmeote machine, but, when I try to join the session from the remote, it just hangs there and will not proceed.  I tried both connection methods, same result.  I can update the guest info just fine, so the "server" side looks like it is good and remains connected to the Connectwise servers.