Related Permissions Extension: Existing Contact widget on Webform not showing related contacts

We are using Webform and the Related Permissions extension to allow parents to register their children for summer camp events. An Existing Contact select widget is included so that parents can select children they have previously registered (which established a permissioned relationship between them).

With "Enforce Permissions" selected on the webform component, the drop down list does not work - no contacts are included in the list, even though the user is logged and has related and permissioned contacts.

When "Enforce Permissions" is de-selected, the drop down list shows the first 500 contacts as expected.

The logs show the following error:

The CiviCRM "contact get" API returned the error: "DB Error: unknown error" when called by function "_webform_render_civicrm_contact" on line 387 of contact_component.inc with parameters: "Array ( [is_deleted] => 0 [contact_type] => Individual [check_permissions] => 1 [rowCount] => 500 [sort] => sort_name [version] => 3 ) "

Drupal 7.50
Civi 4.7.11
Webform 7.x-4.14
Related Permissions Extension 1.2 (with latest codebase installed a couple of weeks ago)

Any ideas if this is an issue with our site or a bug?

(Also reported on Stack Exchange: http://civicrm.stackexchange.com/questions/14950/related-permissions-ext...)