I use xfce on Debian GNU/Linux unstable on two machines.
On both of them the button and menu entry for "switch the user" is missing. I am not able to login with xfce to another user without logoff the current one.
Xfce doesn't actually come with multi-session capabilities like that. In days past, gdmflexiserver used to provide that functionality (with gdm I believe), but it has since been deprecated. I believe that lightdm has similar functionality via the "dm-tool switch-to-greeter" command. If you are using lightdm and are talking about the whiskermenu plugin, add that command in the properties of the plugin for user switching.
So you mean this depends on the used login-manager? Siduction use sddm for that. This could be the reason?
So you mean this depends on the used login-manager?
I believe so as it would be the mechanism to offer up different concurrent sessions.
Siduction use sddm for that. This could be the reason?
Sorry, but I have no experience or knowledge of sddm's capabilities. If it does support it, it should work. This would be over and above Xfce's functionality.