A number of Events Calendar users have seen a 404 Page Not Found message included on their templates/page titles when there are no upcoming events. The support team has recommended adding this snippet (http://pastebin.com/gwm5dnFE) to your theme’s functions.php file as a temporary fix.
In my case, this snippet fixed the 404 problem, but had the side effect of breaking (empty white page) some of the templates included in the theme I was working with (Kriesi’s Enfold theme).
The solution, at least for me, was to slightly modify the function in the above snippet to take $template as an argument and then return it. See modified snippet here: http://pastebin.com/eDk2kC4y.