Hi!
Taking http://14.140.153.162:8082/wordpressasp/events/upcoming-events#sthash.Gvp7wI8j.dpuf as an example, it seems you currently have no upcoming events and, following WordPress conventions, The Events Calendar is returning a 404 status. My guess is that either your theme or WPSEO is deciding to change the title to include “Page not found” on that basis.
Things you can explore:
* This thread discusses ways to workaround this so that empty event views don’t return a 404 status
* You could investigate if it is your theme or WPSEO that’s doing this and set up filters to counter it
Does that help or at least give you a starting point?