Ajax error when selecting text editor

Problem/Motivation

Installed the new Editor 7.x-1.1, and also upgraded to Dialog 7.x-2.2 and Entity Embed 7.x-3.0. Editor module fails with an ajax error when attempting to select "CKEditor" from the Text editor dropdown select in the text format pages.

Steps to reproduce

Navigate to an existing text format, choose "CKEditor" from the text editor dropdown, and an error message pops up:

An Ajax HTTP error occurred.
HTTP Result Code 500
Debugging information follows.
Path: /system/ajax
StatusText: Internal Server Error
ResponseText: