My best guess is that you moved the Calendar to a different page after it already had events. The pageid is in the real url that maps to the friendly url for the event details and that is used to enforce security and now it does not match the page that the instance of Event Calendar Pro is on. It is not supported to move Event Calendar Pro after it has events and it it doesn't support showing the it on more that one page because of this. The only way to fix it is to either move it back to the original page or go through all the friendly urls for events in url manager and correct the real url so that it has the current pageid.

Hope that helps,

Joe