For those in need for a solution until Novalnet will post an update, here is what you need to do.

This applies only if you use WooCommerce Vesion 3.6 or above in conjunction with the Novalnet Plugin Version 11.2.12 or below.

1. Connect to you websever using ftp
2. Navigate to the directory /wp-content/plugins/woocommerce-novalnet-gateway/includes
3. Copy the file wc-novalnet-functions.php to your local computer
4. Open the file with a text editor (NOT Word!!, use something like notepad++)
5. Locate the function wc_novalnet_load_template() arround line 256
6. Replace

if ( function_exists( 'wc_get_template' ) ) {
        wc_get_template(
            $file_name, array(
            'default_path' => $directory_path,
            $content_name  => $contents,
            )
        );

with

if ( function_exists( 'wc_get_template' ) ) {
    /*
        wc_get_template(
            $file_name, array(
            'default_path' => $directory_path,
            $content_name  => $contents,
            )
        );
    */
        wc_get_template( $file_name, array( $content_name  => $contents ), '', $directory_path  );

7. Save the file
8. Transfer the file back to you server using ftp to the same location (/wp-content/plugins/woocommerce-novalnet-gateway/includes)
9. Payments will work again

It is nearly 3 weeks now I reported this issue and still no answer. Please remove ‘default_path’ from the array because it is and was not supported at all. This may cause more issue with future woocommerce version.