Relation field not offering selection for non site administrators

Plone Support Add-on Development
,
1

A site administrator and manager can select an object. The others can not. Message is "searching..." instead of displaying a list of objects.

It is necessary that also Contributors can use this field.

I am using Plone 5.1-latest

The field is defined like dexterity TTW configuration would do:

<field name="doku" type="z3c.relationfield.schema.RelationChoice">
  <description/>
  <title>Doku</title>
  <portal_type>
    <element>Document</element>
    <element>News Item</element>
  </portal_type>
</field>

Javascript Debugger says
TypeError: d is undefined ... plone-logged-in-compiled.min.js:14:24544

Any hint where I can start debugging?

2

It might be an idea to check if (all) the javascript are the same for both users.