Thank you for your suggestions. I suppose it probably is some permissions problem. I now granted the "Customize CKEditor appearance" permission for the role Editor and the toolbar is now functional. However it leaves me puzzled why this paprticular permission would trigger whether a user / role is allowed to use the toolbar buttons or not.
Customize CKEditor appearance
Allow users to customize CKEditor appearance.
Either it's a wording problem and granting that permission is the right thing to to – or I now actually have granted permission to customize the CKEditor appearance (as the wording would suggest) and I have a slight security problem However – i'll submit a request for support issue over at drupal.org and edit this post once I know more…
PS: Even thought this question might leave the impression – it's not my first time on drupal. It's just the first time I'm using CKEditor instead of WYSIWYG.