I found it.
The beer is called "Puparch Knall" and brewed by Brewery Luedde in Quedlinburg. Not far from where I grew up.
Here is a description I found http://www.kneipen-suche.com/index.php4?anzeige=42#87
. I tried Google translate, but I had to clean it up a lot.
translated: A term, that has become indispensable in Quedlinburg, is the beer brewed by Lüdde in Quedlinburg. It's called "Puparsh bang". It was born out of the vernacular. It is a brown beer for self-service. Filled into bottles, it is stored for a week where ferments. It's the best brew for father and son. Nothing can happen, since there is almost no alcohol in it. But it tends to cause flatulence, hence it names. It is also on the menu of the brewery restaurant where you can order it w/o blushing and and it goes off the waitstaff's lips very easy.
Interesting beer. I wonder to what extent there is a risk of bottle bombs since it seems to be young beer according to the comment of little alcohol yet fermentaion.