As the developers seem to have abandoned this good plugin, I have found the solution. They made a mistake and forgot to add one simple line.
You have to edit the file woocommerce-simple-product-badge.php
in your plugin folder. Then, replace the lines 48 to 54:
if ( !empty( $title ) ) {
update_post_meta( $post_id, '_woocommerce_simple_product_badge_title', esc_attr( $title ) );
}
if ( !empty( $class ) ) {
update_post_meta( $post_id, '_woocommerce_simple_product_badge_class', esc_attr( $class ) );
}
With this new code:
if ( !empty( $title ) ) {
update_post_meta( $post_id, '_woocommerce_simple_product_badge_title', esc_attr( $title ) );
} else {
update_post_meta( $post_id, '_woocommerce_simple_product_badge_title', esc_attr( $title ) );
}
if ( !empty( $class ) ) {
update_post_meta( $post_id, '_woocommerce_simple_product_badge_class', esc_attr( $class ) );
} else {
update_post_meta( $post_id, '_woocommerce_simple_product_badge_class', esc_attr( $class ) );
}