Wrong Slug Returned when Form Submitted

I had a GF form creating CPT posts working great last year. But over the past year of plugin and WP updates, and now reactivating this form, I receive a 404 error when the form is submitted. The form does not confirm and the custom post is not created.

From my server error log:
[Fri Apr 25 06:44:41 2014] [error] [client 208.104.128.182] File does not exist: /var/www/vhosts/domain.com/httpdocs/form-slug, referer: http://www.domain.com/form-slug/

The problem is, somewhere in the process, there’s a function pulling “httpdocs” but I’m not sure where.

I’ve deactivated all plugins (except GF 1.8.7 and GFCPT 3.0.1) and switched to a generic Genesis child theme (but added my CPT code), but the error remains.

Ordinary GF forms that do not map to my custom post type work fine.
I tried flushing rewrite rules, and resetting permalinks, but neither helped.

This is a sub-directory multisite network with this form on site ID=1. There are no Capital letters in the cpt slug.

One other thing to note: If I change permalinks back to default, everything works again. But for permalinks = Post name, we get the 404.

I’m looking for advice about how to track down what’s now breaking this for me.

Thanks.

Here is how I’m registering the CPT.

add_action( 'init', 'rbc_register_post_types' );

function rbc_register_post_types() {
    global $blog_id;

    //register custom post types
  rbc_register_post_type( 'Person', 'People', 'people', 'people', false, array( 'title', 'editor', 'thumbnail', 'excerpt', 'author', 'custom-fields' ) );
}

function rbc_register_post_type( $type, $types, $cpt_name, $slug, $search=false, $supports, $showui=true ){

  $type2=strtolower( $type );
  $types2=strtolower( $types );
  $slug=str_replace( " ", "-", $types2 );
  $labels = array(
  'name' =>$types,
  'singular_name' => $type,
  'add_new' => 'Add New',
  'add_new_item' => 'Add New ' .$type,
  'edit_item' => 'Edit '.$type,
  'new_item' => 'New '.$type,
  'view_item' => 'View '.$type,
  'search_items' => 'Search '.$types,
  'not_found' =>  'No '.$types2.' found',
  'not_found_in_trash' => 'No '.$types2.' found in Trash',
  'parent_item_colon' => '',
  'menu_name' => $types
  );

  $args = array(
  'labels' => $labels,
  'public' => true,
  'publicly_queryable' => true,
  'show_ui' => $showui,
  'query_var' => true,
  'rewrite' => array("slug" => $slug, "with_front" => false),
  'capability_type' => 'post',
  'hierarchical' => false,
  'menu_position' => null,
  'has_archive' => true,
  'exclude_from_search' => $search,
  'supports' => $supports
  );

  register_post_type( $cpt_name, $args );
}

https://ww.wp.xz.cn/plugins/gravity-forms-custom-post-types/