500 Internal Server Error (Ajax) when adding existing entity.

Problem/Motivation

Prior to upgrading from v8.x-1.0-rc17, IEF worked fine. For D11 compatibility we are attempting to upgrade to v3.0.0-rc20. With v3 we can add a new entity (via IEF) but when we select an existing entity we get a POST 500 Internal Server Error having to do with Ajax. We are unable to identify what exactly is failing.

Steps to reproduce

Create a new content type and add a new field "Reference" field. Select "Content" for the entity reference type and configure the field to accept and existing content type. Next select the "Manage form display" tab and edit the new reference field to use "Inline entity form - Complex" and "Allow users to add existing nodes".

Create a new page using the new content type and for the new entity reference field, click the "Add existing node" button. Use type-ahead to select an existing node and click the "Add node" button. nothing happens and errors are displayed in the browser console.

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes