Hi @delaner,
PDF invoices cannot be opened properly on some servers due to an error log conflict. So please added below code on your wp-config.php. The query plugin also follows this method. that’s why it will work when you activate it.
@ini_set( ‘log_errors’, ‘On’ );
@ini_set( ‘display_errors’, ‘Off’ );
define( ‘WP_DEBUG’, true );
define( ‘WP_DEBUG_LOG’, true );
define( ‘WP_DEBUG_DISPLAY’, false );