Hi Silvan,
This is what appears in the terminal :

mars 30 20:15:09 openmamba dbus-daemon[6819]: [session uid=1000 pid=6819] Successfully activated service 'org.kde.LogoutPrompt'
mars 30 20:15:09 openmamba ksmserver-logout-greeter[8068]: kde.logout_greeter: Couldn't find a theme for the Shutdown dialog ""

Hi,
this seems like a theme problem. Are you using the default theme? If so, can you try to reapply it from the Appearance > Global theme page in the settings? It is not necessary to check the box for reapplying also the dispositions so that you won't lose your customization.

Hello Silvan,

With OpenMamba default theme, deconnection screen appears fine.

Thanks for your help.