Spent 2 days on this similar problem. Sebastian, you almost had it. Twig snippet should look more like this:
{% for item in items %} <a href="{{ item.content['#url'] }}" class="btn-circle btn-circle--blue js-open-modal"> {{ item.content.[#title']['#title'] }} </a> {% endfor %} {# [#title']- this is really important to place the text in the a href tag #} ['#title'] - is really important to place the text in the a href tag
Definitely MortenDK was very helpful: https://www.youtube.com/watch?v=bvaz9eb54c4