How to configure Entity Reference View Widget

Thank you for this, I created a documentation page (https://www.drupal.org/node/2305545), feel free to reedit it if you're up to :).

I get this everytime I try to open the entity view widget field:

An AJAX HTTP error occurred.
HTTP Result Code: 404
Debugging information follows.
Path: /system/ajax
StatusText: Not Found
ResponseText:
Page not found | Boardplan
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etc - any help would be greatly appreciated.