The default User/SideBar does not contain a Logout option following setup and data migration from BlueSpice v3. Custom user Sidebars are also affected.
The User Sidebar should have a Logout option in the Personal Tools section (as per the Demo Wiki at Demo BlueSpice, but this is not present in two instances that I have deployed. Browsing to the Special:UserLogout page allows the user to logout.
Do you have any authentication plugin enabled? Extension:PluggableAuth, Extension:SimpleSAMLphp, Extension:OpenIDConnect, Extension:LDAPAuthentication2, Extension:Auth_remoteuser ?