A bug was discovered in the WP-Members 2.3.1 code improvement release. In the admin panel, scrolling down to the very bottom, you will see the following error message:
Fatal error: Call to undefined function wpmem_admin_users_table() in /home/license/public_html/wp-content/plugins/wp-members/wp-members-admin.php on line 364
This is a call to a function that is currently being tested and was not to be included in the 2.3.1 release.
I will correct this bug in the next update. Since it does not effect the functioning of the plugin or of WordPress itself, I will hold off on an immediate release in the (hopefully unlikely) event that some other bugs surface during that time.
In the meantime, if you feel the need, you can correct it by opening the wp-members-admin.php file and removing the following line (364):
<?php wpmem_admin_users_table(); ?>
This can be done from the WP admin panel under Plugins > Editor.
Thanks to the users who pointed this out (since it is in a less than noticeable location)!