Help! Loads of warning messages!

Hi,
This feature used to work well, and now all of a sudden there are lines and lines of warnings – apparently the same line repeats itself several times – at the top of the page as well as below the title bar:

Warning: Use of undefined constant km – assumed ‘km’ (this will throw an Error in a future version of PHP) in /home/.sites/20/site4607206/web/wp-content/plugins/members-list-pro/class/members.php on line 730

How can I get rid of that?
Thanks!!

The page I need help with: [log in to see the link]