Everything appears to be setup and working correctly - Veyon 4.2.2, testing on two Win10 machines, got keys/specific user groups working don't mind which I use. Computers show up and Remote View/Control works, can't get any other features to work at all.
I can also add that the newer version I am testing here is working perfectly to control PCs in our ICT Suite that are on an old version 4.0.* and running Windows 8.1.
That's an interesting observation! Are you using the key file authentication? Can you try to change the update interval to a lower value? Is any kind of access control configured?