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.
ckeditor_permission()
that there's no "access ckeditor". The way text formats and profiles are set up it wouldn't make much sense for there to be. The text format permissions should be all that's required