IEF hangs and AJAX error occurs when clicking Add New Variation

There is nothing I can do to debug that.
You will have to return with more information.

I'm having a similar issue. I'm having a hell of a time debugging it because of a lack of error detail.

When I click Add New Variation, Please Wait appears and the AJAX post to system/ajax aborts and throws an alert with a very general message. (No error in PHP log)

An AJAX HTTP request terminated abnormally.
Debugging information follows.
Path: /system/ajax
StatusText: error
ResponseText:
ReadyState: 0

I have no extra fields in my Product type, other than the default SKU, Title, Price and Status. One important note - Add New Variation works as expected if I create a second Product type and limit "Product types that can be referenced" to the 2nd product. It breaks if I reference the same Product type. Is this an issue with self referencing?

Module versions
Commerce: 7.x-1.7
Entity: 7.x-1.0
Inline Entity Form: 7.x-1.1 (also tried latest dev version)

Here is the Form Data that causes the system/ajax post to abort:

sku:
title:
commerce_price[und][0][amount]:
form_build_id:form-mfvqCwLgm5Zy4TqT5n5xmy_jUnO44fj2DZ0ANLLfxp4
form_token:HJ_ai1nbfPzn33JTr94H8WQ-UIwevpk5FBt-Yku07Pw
form_id:commerce_product_ui_product_form
status:1
field_product_variations[und][actions][bundle]:test
log:
_triggering_element_name:ief-116-add
_triggering_element_value:Add new variation
ajax_html_ids[]:skip-link
ajax_html_ids[]:branding
ajax_html_ids[]:page-title
ajax_html_ids[]:page
ajax_html_ids[]:main-content
ajax_html_ids[]:content
ajax_html_ids[]:block-system-main
ajax_html_ids[]:commerce-product-ui-product-form
ajax_html_ids[]:edit-sku
ajax_html_ids[]:edit-title
ajax_html_ids[]:edit-commerce-price
ajax_html_ids[]:commerce-price-add-more-wrapper
ajax_html_ids[]:edit-commerce-price-und-0-amount
ajax_html_ids[]:edit-status
ajax_html_ids[]:edit-status-1
ajax_html_ids[]:edit-status-0
ajax_html_ids[]:edit-field-product-variations
ajax_html_ids[]:inline-entity-form-116
ajax_html_ids[]:edit-field-product-variations-und
ajax_html_ids[]:edit-field-product-variations-und-actions
ajax_html_ids[]:edit-field-product-variations-und-actions-bundle
ajax_html_ids[]:edit-field-product-variations-und-actions-ief-add
ajax_html_ids[]:edit-change-history
ajax_html_ids[]:edit-log
ajax_html_ids[]:edit-actions
ajax_html_ids[]:edit-submit
ajax_html_ids[]:edit-save-continue
ajax_html_ids[]:footer
ajax_html_ids[]:admin-menu
ajax_html_ids[]:admin-menu-wrapper
ajax_html_ids[]:admin-menu-icon
ajax_html_ids[]:admin-menu-menu
ajax_html_ids[]:admin-menu-account
ajax_page_state[theme]:rubik
ajax_page_state[theme_token]:-qdi09EtuXoPM7Y7c1AVg3UCjTxbTMJwc_WxzhT2faA
ajax_page_state[css][misc/ui/jquery.ui.core.css]:1
ajax_page_state[css][misc/ui/jquery.ui.theme.css]:1
ajax_page_state[css][modules/field/theme/field.css]:1
ajax_page_state[css][sites/all/modules/views/css/views.css]:1
ajax_page_state[css][sites/all/modules/admin_menu/admin_menu.css]:1
ajax_page_state[css][sites/all/modules/admin_menu/admin_menu.uid1.css]:1
ajax_page_state[css][sites/all/modules/admin_menu/admin_menu_toolbar/admin_menu_toolbar.css]:1
ajax_page_state[css][modules/shortcut/shortcut.css]:1
ajax_page_state[css][sites/all/modules/ctools/css/ctools.css]:1
ajax_page_state[css][sites/all/modules/panels/css/panels.css]:1
ajax_page_state[css][sites/all/modules/tagadelic/tagadelic.css]:1
ajax_page_state[css][sites/all/modules/context/plugins/context_reaction_block.css]:1
ajax_page_state[css][sites/all/modules/commerce/modules/price/theme/commerce_price.theme.css]:1
ajax_page_state[css][sites/all/modules/inline_entity_form/theme/inline_entity_form.css]:1
ajax_page_state[css][sites/all/modules/inline_entity_form/theme/commerce-product.css]:1
ajax_page_state[css][sites/all/modules/field_group/field_group.field_ui.css]:1
ajax_page_state[css][sites/all/modules/views/css/views-admin.seven.css]:1
ajax_page_state[css][sites/all/themes/tao/reset.css]:1
ajax_page_state[css][sites/all/themes/tao/base.css]:1
ajax_page_state[css][sites/all/themes/tao/drupal.css]:1
ajax_page_state[css][sites/all/themes/rubik/core.css]:1
ajax_page_state[css][sites/all/themes/rubik/icons.css]:1
ajax_page_state[css][sites/all/themes/rubik/style.css]:1
ajax_page_state[css][sites/all/themes/rubik/print.css]:1
ajax_page_state[js][sites/all/modules/jquery_update/replace/jquery/1.7/jquery.min.js]:1
ajax_page_state[js][misc/jquery.once.js]:1
ajax_page_state[js][misc/drupal.js]:1
ajax_page_state[js][sites/all/modules/jquery_update/replace/ui/ui/minified/jquery.ui.core.min.js]:1
ajax_page_state[js][sites/all/modules/jquery_update/replace/ui/ui/minified/jquery.ui.widget.min.js]:1
ajax_page_state[js][sites/all/modules/jquery_update/replace/ui/ui/minified/jquery.ui.mouse.min.js]:1
ajax_page_state[js][sites/all/modules/jquery_update/replace/ui/ui/minified/jquery.ui.draggable.min.js]:1
ajax_page_state[js][sites/all/modules/jquery_update/replace/ui/ui/minified/jquery.ui.droppable.min.js]:1
ajax_page_state[js][sites/all/modules/jquery_update/replace/ui/ui/minified/jquery.ui.sortable.min.js]:1
ajax_page_state[js][sites/all/modules/jquery_update/replace/ui/external/jquery.cookie.js]:1
ajax_page_state[js][sites/all/modules/jquery_update/replace/misc/jquery.form.min.js]:1
ajax_page_state[js][misc/form.js]:1
ajax_page_state[js][misc/ajax.js]:1
ajax_page_state[js][sites/all/modules/admin_menu/admin_menu.js]:1
ajax_page_state[js][sites/all/modules/admin_menu/admin_menu_toolbar/admin_menu_toolbar.js]:1
ajax_page_state[js][sites/all/modules/panels/js/panels.js]:1
ajax_page_state[js][sites/all/modules/context/plugins/context_reaction_block.js]:1
ajax_page_state[js][misc/progress.js]:1
ajax_page_state[js][misc/collapse.js]:1
ajax_page_state[js][sites/all/modules/inline_entity_form/inline_entity_form.js]:1
ajax_page_state[js][sites/all/themes/rubik/js/rubik.js]:1

Check if there is anything in watchdog or your error logs.

Nothing in watchdog or php error logs.

Ok, I've figured this out. I was attempting to use a product reference field within the Product entity rather than a node. I guess referencing a product entity from another product entity isn't supported. I've moved the product reference with inline entity form widget to my product content type (node) and everything is working as expected.

Yes, that's currently not supported, there's an issue in the queue about fixing it.