Who's online links go to LaLa land ...and never load
 

I run a few forums and have never seen this issue before. When I click on Who's online it appears to be trying to load the page but never does. Whatever page I'm on when I click the link stays in place until I click stop. Other users are experiencing this problem also.
Both the drop down quick links who's online and the who's online link at the bottom with Currently active users give the same non result.
No new plug ins added for months.
Any ideas what might fix this?