Add Events Broken?

Hi All. This looks like a promising plugin but when I select ‘Add Events’, I get the following message. Any Ideas?.

Internal Server Error

The server encountered an internal error or misconfiguration and was unable to complete your request.

Please contact the server administrator, [email protected] and inform them of the time the error occurred, and anything you might have done that may have caused the error.

More information about this error may be available in the server error log.

Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

The server error log contains numerous related errors:

client 95.150.252.118] File does not exist: /home/xxxxx/public_html/500.shtml, referer: https://xxxxx.co.uk/harrierstriwordpress/wp-admin/plugins.php?activate=true&plugin_status=all&paged=1&s=
[Sat Feb 22 09:56:50 2014] [error] [client 95.150.252.118] File does not exist: /home/moorene/public_html/500.shtml, referer: https://moorenet.co.uk/harrierstriwordpress/wp-admin/edit.php?post_type=calp_event
[Sat Feb 22 09:55:37 2014] [error] [client 95.150.252.118] File does not exist: /home/xxxxxx/public_html/500.shtml
[Sat Feb 22 09:55:32 2014] [error] [client 95.150.252.118] File does not exist: /home/xxxxxx/public_html/500.shtml, referer: https://moorenet.co.uk/harrierstriwordpress/wp-admin/edit.php?post_type=calp_event&page=calpress-event-calendar-settings
[Sat Feb 22 09:55:20 2014] [error] [client 95.150.252.118] File does not exist: /home/xxxxx/public_html/500.shtml, referer: https://xxxxxx.co.uk/harrierstriwordpress/wp-admin/edit-tags.php?taxonomy=events_categories&post_type=calp_event
[Sat Feb 22 09:55:01 2014] [error] [client 95.150.252.118] File does not exist: /home/xxxxxx/public_html/500.shtml, referer: https://xxxxxco.uk/harrierstriwordpress/wp-admin/edit.php?post_type=calp_event
[Sat Feb 22 08:47:48 2014] [error]

https://www.ads-software.com/plugins/calpress-event-calendar/