Help! Calendar broken - Yosemite

[nealeholl]

I received an invitation from someone, which I attempted to put in my Mac Calendar; I don't know what app the invitation was generated by, but my Calendar now opens with a great long pop-up script starting "The server responded with an error". There are no "traffic lights" to close the pop-up, and I can't use Calendar while it's there. The only way to get rid of it is to use Force Quit > Calendar, which closes it all down, but it reappears as soon as I relaunch Calendar. Any ideas??