This issue turned out to most likely be related to the Shinobi charts used in the app. The app polls a webservice with weather data. Sometimes the data can be invalid and this was not handled correctly in the app. When invalid data was present the chart was not able to render in a new UIViewController and crashed the app.
This bug was somewhat hard to find, particularly because of the strange, amputated crash report in google analytics.
Another possible explanation for the crash could be that it was possible for IOS5 users to install the app. The app would then not start because "Autolayout" which needs at least iOS6 was used...
iTunes Connect still does not contain a crash report. The app has about 200 users.