Demo Mode / black screen

[carlbosch]

Hello,
I am trying to get the demo mode to work. I want to distribute the screen of a teacher PC to the student PCs using the demo function. I only get a black screen on the student PCs.
Other functions of Veyon work.
We use a key pair for authentication and work with LDAP.
The description in the admin documentation on the subject of demo mode - black screen was unsuccessful.
We use Windows 10 computers.
I get the following messages in the VeyonMaster.log file:

2024-06-14T15:04:09.704: [DEBUG] VncFeatureMessageEvent::fire(): 2024-dell-o:11100 FeatureMessage(FullScreenDemo,3,{})
2024-06-14T15:04:09.704: [DEBUG] VncFeatureMessageEvent::fire(): 2024-dell-o:11100 FeatureMessage(WindowDemo,3,{})
2024-06-14T15:04:09.704: [DEBUG] VncFeatureMessageEvent::fire(): 2024-dell-o:11100 FeatureMessage(FullScreenDemo,3,{})
2024-06-14T15:04:09.704: [DEBUG] VncFeatureMessageEvent::fire(): 2024-dell-o:11100 FeatureMessage(WindowDemo,3,{})
2024-06-14T15:04:09.704: [DEBUG] VncFeatureMessageEvent::fire(): 2024-dell-o:11100 FeatureMessage(FullScreenDemo,3,{})
2024-06-14T15:04:09.704: [DEBUG] VncFeatureMessageEvent::fire(): 2024-dell-o:11100 FeatureMessage(WindowDemo,3,{})
2024-06-14T15:04:09.704: [DEBUG] VncFeatureMessageEvent::fire(): 2024-dell-o:11100 FeatureMessage(FullScreenDemo,3,{})
2024-06-14T15:04:09.704: [DEBUG] VncFeatureMessageEvent::fire(): 2024-dell-o:11100 FeatureMessage(WindowDemo,3,{})
2024-06-14T15:04:09.704: [DEBUG] VncFeatureMessageEvent::fire(): 2024-dell-o:11100 FeatureMessage(FullScreenDemo,3,{})
2024-06-14T15:04:09.704: [DEBUG] VncFeatureMessageEvent::fire(): 2024-dell-o:11100 FeatureMessage(WindowDemo,3,{})
2024-06-14T15:04:09.704: [DEBUG] VncFeatureMessageEvent::fire(): 2024-dell-o:11100 FeatureMessage(ScreenLock,1,{})
2024-06-14T15:04:09.704: [DEBUG] VncFeatureMessageEvent::fire(): 2024-dell-o:11100 FeatureMessage(MonitoringMode,1,{"4":5000})
2024-06-14T15:04:09.705: [DEBUG] VncFeatureMessageEvent::fire(): 2024-dell-o:11100 FeatureMessage(WindowDemo,2,{"0":"��2Y8?.��\u0017�D�Um9\u0000s��hH����DB)�\u0004�+\u0017!���i"�a��l,[���6>6�!��\u0014�j�)b6ʋ(@��4p�m�xe\u0012�\u000b=2gG�E���0��b�牳`~�jۙe�Qq�E\u001f\u0013�-���\u0003cbkO��\b[�˥","3":"","4":11400,"5":null})
2024-06-14T15:04:10.711: [DEBUG] VncConnection::rfbClientLogDebug(): 0x367c VNC server supports protocol version 3.8 (viewer 3.8)
2024-06-14T15:04:10.713: [DEBUG] VncConnection::rfbClientLogDebug(): 0x367c We have 1 security types to read
2024-06-14T15:04:10.713: [DEBUG] VncConnection::rfbClientLogDebug(): 0x367c 0) Received security type 40
2024-06-14T15:04:10.713: [DEBUG] VncConnection::rfbClientLogDebug(): 0x367c Selecting security type 40 (0/1 in the list)
2024-06-14T15:04:10.713: [DEBUG] VncConnection::rfbClientLogDebug(): 0x367c Selected Security Scheme 40
2024-06-14T15:04:10.714: [DEBUG] VeyonConnection::handleSecTypeVeyon(): 0x367c received authentication types: QList(RfbVeyonAuth::KeyFile)
2024-06-14T15:04:10.714: [DEBUG] VeyonConnection::handleSecTypeVeyon(): 0x367c chose authentication type: QList(RfbVeyonAuth::KeyFile)
2024-06-14T15:04:10.716: [DEBUG] WtsSessionManager::querySessionInformation(): 2 WtsSessionManager::SessionInfo::UserName "XXXXXXXX"
2024-06-14T15:04:10.718: [DEBUG] WtsSessionManager::querySessionInformation(): 2 WtsSessionManager::SessionInfo::DomainName "XXXX-XXXX-XX"
2024-06-14T15:04:10.730: [DEBUG] VncConnection::rfbClientLogDebug(): 0x367c VNC server closed connection

It looks like the teacher PC cannot open a VNC connection.
Does anyone have any idea what the problem could be?