What are the most common prerequisites demanded by Promotionsausschüssen (German PhD boards)?
Other requirements (not necessarily prerequisites) may include the attendance of doctoral seminars (i.e. presenting your work one or more times during your PhD and having a committee judge whether or not you can proceed as intended) and the presentation of a certificate of good conduct ("Führungszeugnis"). 

Source: http://www.inf.ovgu.de/inf_media/downloads/forschung/promotion/2013/Promotionsordnung_2012_12_19-p-4194.pdf