The problem is with the multiple logged in users in your system. I used to experience the same problem. Solved it by a policy chane allowing logout even when multiple users are logged in. If you understand the implications(like lose of non-saved data, for other logged in users) of allowing such a thing, you could go ahead and do the same.
It can be done as follows:
sudo vi /etc/polkit-1/localauthority/50-local.d/usershutdown.pkla
As content of that file, enter the following and save it:
Now restart your system and check if things are working as intended.
N.B: You could use any other editor of your choice to create that file.