Typically, this is the result of one corrupt Source Calendar. You will need to do process of elimination in order to determine which source calendar is the culprit. You may consider creating a copy of the page and a new part to test. The overview of the process is as follows, after making the copy page and new part:
  1. Open the Original Part in one window and the New Part in another window, both in edit mode
  2. In the New Part, under Pick Source Calendars, mark the box for Display for a calendar that is on the original part
  3. Click Save
  4. Check the copy page to see if the error occurs
  5. Repeat until the error is generated and make note of the Calendar that prompted this error
Once you determine which Calendar is the culprit, re-create this Calendar, and add the new Calendar to the Event Calendar Group. 

Run time error accessing particular page