Problem with Raspberry

[video68]

Hi, I've installed Veyon client on 2 Raspberry pi 3 (joined in a debian domain). The main window of veyon master (Windows 10) works fine but when try to connect to the client, the connection disappears from the main window (become gray, then red) and the window of the connection doesn't show the desktop of the client but only the message Connection with...
If I close the connection window, the desktop of the client come back after 1-2 minutes.

Sorry for my english and thank you in advance for you reply
Matteo