I tried to send this as a PM but since I'm new to the forum it wouldn't let me until I posted something so here we go!
Were you able to find a resolution to your RealVNC Cloud Connect issue? We have started having the same issue in the past couple weeks. I've been working with RealVNC on the issue and they asked me to upgrade to 6.2.1 but that didn't resolve the issue. I didn't make any changes to Sonicwall, however, sometimes automated updates may change things. I saw your post from November and hoped you may have found the solution. Thank you!