Even when you connect via remote desktop, it is just a translation layer to VNC. Nothing wrong with VNC.
There is nothing wrong with VNC itself, no need to get defensive about the protocol.
But in this instance it seems that the translation RDP to VNC is working a lot better than the native VNC connection, assuming that extension is the native connector.
On the other hand why does RDP still work if I remove the extension? That means its either not the native VNC connector, or the RDP implementation is every bit as native as VNC to the VBox server.
The extension also contains the following libs:
So in fact this looks a lot more like an additional extension pack to me than the other way round.
Well, I got a solution that works for me. If an update restores the extension pack and it stops working, I know where to look.