Logon prompt does not appear after ctrl-alt-delete on Windows Server 2019

[fkamp]

I ran into an issue where all our Windows Server 2019 installations dont work with UltraVNC anymore. Establishing the VNC session works fine, however, after sending ctrl-alt-delete for SAS, the login prompt does not appear. It only shows a blue screen. After some time, it will revert back to showing "Press ctrl-alt-delete to unlock".

Windows Server 2019 Standard
OS build 17763.5576

UltraVNC 1.4.3.6

Does anyone have an idea?

Here is an excerpt of the logfile:

Fri Mar 22 16:55:55 2024
vncDesktopThread::run_undetached : Hook changed 
 --The requested resource is in use.
Fri Mar 22 16:56:00 2024
vncEncodeMgr::SetEncoding : unknown encoder requested: 24
vncEncoder::SetTranslateFunction : only 8, 16 or 32 bits supported remotely - 0 requested
Fri Mar 22 16:56:04 2024
 keysym 0xffe9vncMenu::~vncMenu : vncmenu killed
 --The requested resource is in use.
WinVNCAppMain : ################## Closing Imp Thread
imp_desktop_thread : OpenInputdesktop OK
imp_desktop_thread : SelectHDESK to Winlogon (4f4) from 100
imp_desktop_thread : Username SYSTEM 
vncMenu::vncMenu : vncmenu(server)
 keysym 0xff09Fri Mar 22 16:56:05 2024
 keysym 0xff09 keysym 0xffe9Fri Mar 22 16:56:20 2024
vncMenu::~vncMenu : vncmenu killed
 --The requested resource is in use.
WinVNCAppMain : ################## Closing Imp Thread
imp_desktop_thread : OpenInputdesktop OK
imp_desktop_thread : SelectHDESK to Winlogon (460) from 100
imp_desktop_thread : Username SYSTEM 
vncMenu::vncMenu : vncmenu(server)
Fri Mar 22 16:56:29 2024
 keysym 0xffe3 keysym 0xffe9 keysym 0xffff keysym 0xffff keysym 0xffe9 keysym 0xffe3Fri Mar 22 16:56:37 2024
vncMenu::~vncMenu : vncmenu killed
 --The requested resource is in use.
WinVNCAppMain : ################## Closing Imp Thread
imp_desktop_thread : OpenInputdesktop OK
imp_desktop_thread : SelectHDESK to Winlogon (434) from 100
imp_desktop_thread : Username SYSTEM 
vncMenu::vncMenu : vncmenu(server)
Fri Mar 22 16:56:53 2024
vncMenu::~vncMenu : vncmenu killed
 --The requested resource is in use.
WinVNCAppMain : ################## Closing Imp Thread
imp_desktop_thread : OpenInputdesktop OK
imp_desktop_thread : SelectHDESK to Winlogon (410) from 100
imp_desktop_thread : Username SYSTEM 
vncMenu::vncMenu : vncmenu(server)

[RudiDeVos]

Is vnc running as service ? Only a service can use SAS.

Please verify, on some systems this is not enabled

On some system the software ctl-alt-del is disabled
Using gpedit.msc

Search EnableSecureCredentialPrompting on google for it

[fkamp]

Is vnc running as service ? Only a service can use SAS.

Please verify, on some systems this is not enabled

On some system the software ctl-alt-del is disabled Using gpedit.msc

Search EnableSecureCredentialPrompting on google for it

I should have mentioned that it worked when the server was set up initially. VNC runs as a Windows Service. The blank blue screen comes after sending ctrl-alt-delete. So yes, the SAS prompt is enabled. Thanks for the tip anyway.