Thank you for your post.
I am unable to replicate the issue under FileMaker Pro 14.0.1 under Mac OS X 10.10.4. This is what I did:
1. On a new layout using the "Enlightened" theme, I created a Button Bar with three options: "Übersicht", "Stückliste" and "Debugging".
2. I assigned the same graphic icons you selected for the three buttons.
3. I had each button execute the same script "TEST", which only had the single step "Show Custom Dialog".
4. I set anchors on the left and right of the Button Bar so it would auto-size based on the width of the window.
5. I went to Browse Mode to verify the Button Bar displayed properly and the buttons executed the script.
6. In Layout mode, I selected the first button "Übersicht", and inserted two more buttons: "Dateline" and "Firmen".
7. I assigned the same graphic icons you selected.
8. The two new buttons also executed the same script "TEST".
9. I went to Browse Mode and everything verified properly.
Let me know what I'm doing differently so I can replicate the issue.