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The problem is the variable $default_args is used without being defined in the function EM_Calendar::output (this started happening in version 6.4.8).

Until @msykes fixes this, I have temporarily fixed this by making sure $default_args was set before being used. I changed wp-content/plugins/events-manager/classes/em-calendar.php line 472 from this:

$args['calendar_size'] = in_array($args['calendar_size'], $allowed_sizes) ? $args['calendar_size'] : $default_args['calendar_size']; //large will sort itself out

To this:

$default_args = self::get_default_search(array());
$args['calendar_size'] = in_array($args['calendar_size'], $allowed_sizes) ? $args['calendar_size'] : $default_args['calendar_size']; //large will sort itself out