The issue with the preferences under non-English locales (specifically German) on OS X was fixed a month or so ago; it has been in the nightly builds ever since and will be in the next release.
The workaround for now is to switch your system to English, run Ardour and in Preferences > User Interaction change “Use Translations …” to be disabled. Then flip your system back to German. The problem was caused by some illegal character that crept into the German translation. We have been doing major development work since, and did not want to make a hotfix release for this issue.
Regarding your comments on indispensability: I have been working on Ardour for 16 years. During that time, I’ve met audio engineers from all over the world, ranging from 20th Century Fox post-production studios, to high end studios across the US and Europe, to home studio owners, to the owners and developers of many other audio technology companies. No matter what your particular experience of the need for vertical zoom is, I can absolutely assure you that your view is not shared by everyone who does audio engineering. I understand that you feel an absolute need for it, and I respect that. I get to see a much broader selection of DAW users than most DAW users themselves do, and I have a different perspective on its criticality.
I’m not saying that it would not be nice to have for various reasons.