Steps to reproduce the issue

Using either the new Users > Logout/Login menu item or the Login module
Set Logout Redirection Page to any but Default
Login through the front of the Joomla site
click Logout

Expected result

On Logout to be redirected to the selected page

Actual result

It sticks on an empty com_users page index.php?option=com_users&task=user.logout&3376217a04a46c9f4d1133aeb59ae76e=1&return=aW5kZXgucGhwP0l0ZW1pZD0xMDE=

System information (as much as possible)

dbversion: 10.0.26-MariaDB
dbcollation: latin1_swedish_ci
dbconnectioncollation: utf8mb4_general_ci
phpversion: 5.6.24
version: Joomla! 3.6.0 Stable [ Noether ] 12-July-2016 21:14 GMT
platform: Joomla Platform 13.1.0 Stable [ Curiosity ] 24-Apr-2013 00:00 GMT
useragent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:47.0) Gecko/20100101 Firefox/47.0

Additional comments

Looking at UsersControllerUser (components/com_users/controllers/user.php) logout function and base64_decode($return) on line 162 is "index.php?Itemid=101" which is not is_numeric Line 165 so the redirect (Line 204 to 208) is never triggered.