Maybe these two strings: Failed to switch users. Switch _Users Should be: Failed to switch user. Switch _User Without the "s" at the end of "users". xfce4-session is using "user" not "users".
Graeme Gott referenced this bugreport in commit fae01b6e1bb35a6fd631919886cf5386b1d1ec6b Adjust wording of switch user command. (bug #15398) https://git.xfce.org/panel-plugins/xfce4-whiskermenu-plugin/commit?id=fae01b6e1bb35a6fd631919886cf5386b1d1ec6b