Relation field not offering selection for non site administrators

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">
    <element>News Item</element>

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

Any hint where I can start debugging?

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