No event_number

I am looking for help in debugging a curious issue in qem. I have a production site where everything works fine. I copied that site (database and files) to a new site, updated the URLs etc., and now get a warning message when displaying an agenda view of events, namely:

Warning: A non-numeric value encountered in /home/blahblah/wp-content/plugins/quick-event-manager/quick-event-manager.php on line 983

It turns out that almost all of my events have no limit to the attendees and thus no registrants. So there are no rows in wp_postmeta for the events where meta_key = ‘event_number’, so the function get_post_meta($pid, ‘event_number’, true); does not return a numeric value, hence the warning.

If I add to the wp_config.php file:

define( ‘WP_DEBUG_DISPLAY’, false );
@ini_set( ‘display_errors’, 0 );

the warnings go away.

So here is my question. Can you account for why the warnings are displayed, whereas a) in an identical site (except for the hostname), they are not displayed; and b) no other php warnings are displayed anywhere else on the site if I do not add those two lines to my config file? I know for sure there are plenty of deprecated and otherwise dubious practices in the other plugins I use.