/rest/api/3/groupuserpicker in AP.request context, doesn't return users and groups for some users

In an iframe context I try to get the users and groups of the currently logged in user with:

                url: '/rest/api/3/groupuserpicker',

                data: {
                    maxResults: MAX_QUERY_RESULTS,
                    caseInsensitive: true,
                    excludeConnectAddons: true,
                    showAvatar: true

                success: data => successFn(data, this),
                error: error => console.error(error)

That works when the logged in user is the admin user, but returns no users or groups for a non admin user. Despite this user can choose new assignees in an issue view.

I get not an error back. Just the normally returned object with an empty array.

Why could this be?
APP Scope READ is defined.
Are there any other permissions necessary to get the list in an iframe context?

Hello @m.schmidt,

Kindly check if the user is part of a group with Browse user and groups Global permission. If the user does not have this global permission, an empty result set will be returned.