What also would help for debugging are the following steps:
A) before upgrading, change the VNC server backend to Headless VNC server - if the reboot does not happen after upgrading, the issue is related with the updated UltraVNC server component in Veyon 4.10.3
B) before upgrading, disable Veyon service autostart - login should work normally also after upgrading. What happens when starting the service manually after being logged in?
C) Check the Windows event log for indications about why the system has been rebooted
D) In Veyon Configurator in advanced view mode, at page "Windows" there's a setting called "Handling of interfering windows". Is it set to a value other than "None"? If so, please revert it to "None".