Hm, it's hard to say what went wrong, it depends on sooo many factors:
What yeast did you use?
How long was first fermentation?
What was the final alcohol content?
What temperature was the first fermentation?
Did you cold crash it?
What temperature was the beer when kegging?
What sugar did you use?
Did you put the sugar in the beer or the keg?
What temperature was the second fermentation?
How long was the second fermentation?
To be honest, a lager is extremely temperature sensitive because of the yeast. Also if some alcohol content is arrived the yeast will die.
On the other hand I know that these kegs are sold in supermarkets, so the system itself can't be flawed. This why I decided to go with these solution. Okay, professional breweries normally don't use secondary fermentation for carbonating beer, they rather use forced canonisation because it's mach easier to handle in large scale production.
However, also there is a solution. for example you could buy a counter pressure filling unit for these kegs (
http://www.wecomatic.com/5-l-party-kegs/22/filling-unit-for-5-l-party-kegs/cans)
Think the way to go to rescu your Pilsner would be forced carbonation using the party/professional tab with CO2.
Beer brewing is normally a natural process and I would rather try to find the flaw in the process than in the technology used. In Egypt 2000 years ago they had not this kind of technology and got pissed too