Many of GMX's ads seem to be sourced from Taboola.com, which is simply an advertising platform, rather than an actual vendor of goods or services. While many of the businesses placing their ads through Taboola may be legitimate, their platform is also very popular with vendors of so-called 'wonder products', which (for example) claim to be able to save people a fortune on their electricity bills by simply installing a gadget or be able to do away with their satellite TV without losing channels simply by buying a special aerial, etc, etc. All those ads are all no more than con tricks.
So it's not GMX that's failing to monitor the ads but Taboola. Almost any website (or web service) that sources its advertising content externally, rather than directly selling advertising space itself, is likely to display some very dodgy ads from time to time. The Answerbank is definitely no exception. I've lost count of the number of times I've seen dodgy 'save on your electricity' or 'do away with your satellite dish' ads on here.