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'] }} 
 </a>
{% endfor %}

{% for item in items %} 

<a href="{{ item.content['#url'] }}" class="btn-circle btn-circle--blue js-open-modal">{{ item.content.[#title'] }} 
 
 {# [#title']- this is really important to place the text in the a href tag #} 

</a> 

Definitely MortenDK was very helpful: https://www.youtube.com/watch?v=bvaz9eb54c4