AJAX HTTP error clicking 'Cancel' button

Scenario

I have three nested inline entity form like this

- Node entity A
|
-- Node bundle B
|
--- Image entity

I'm using Inline Entity Form - Complex widget allowing to add new entity.

Issue

I click on "Create image" and then on "Cancel" and I get this ajax error:

POST http://mysite.dev/node/add/story?ajax_form=1&_wrapper_format=drupal_ajax&_wrapper_format=drupal_ajax 500 (Internal Server Error)
(anonymous)	@	VM1193:1
send	@	jquery.min.js?v=2.2.4:4
ajax	@	jquery.min.js?v=2.2.4:4
e.fn.ajaxSubmit	@	jquery.form.min.js?v=3.51:11
Drupal.Ajax.eventResponse	@	ajax.js?v=8.3.7:679
(anonymous)	@	ajax.js?v=8.3.7:536
dispatch	@	jquery.min.js?v=2.2.4:3
r.handle	@	jquery.min.js?v=2.2.4:3

If I try to copy&paste that post URL on browser I get this:

[{"command":"add_css","data":"\u003Clink rel=\u0022stylesheet\u0022 href=\u0022\/core\/themes\/classy\/css\/components\/messages.css?ov3ivz\u0022 media=\u0022all\u0022 \/\u003E\n"},{"command":"insert","method":"replaceWith","selector":null,"data":"\n    \u003Cdiv role=\u0022contentinfo\u0022 aria-label=\u0022Error message\u0022 class=\u0022messages messages--error\u0022\u003E\n          \u003Cdiv role=\u0022alert\u0022\u003E\n                  \u003Ch2 class=\u0022visually-hidden\u0022\u003EError message\u003C\/h2\u003E\n                    An unrecoverable error occurred. The uploaded file likely exceeded the maximum file size (32 MB) that this server supports.\n                \u003C\/div\u003E\n      \u003C\/div\u003E\n    ","settings":null}]