I have secured my wp-admin directory by htaccess so only I can use this directory (IP block). Because the Ajax Event Calendar allows users to create events a call is made to wp-admin\admin-ajax.php
This results in server errors (access denied by server configuration).
I think you should move the admin-part of AEC to another place. For the time being I have commented out line 576 in ajax-event-calendar. I don't want to let people add events, so for me that works.