I'm not sure if it's fair to call it rigging, but there was a massive smear campaign against a judge nominated for their constitutional court. Leading to the nomination being withdrawn when it really should've been an appointment as usual. Which is likely the first massive step toward Germany politicising one of the foundations of their democracy, similar to how the USA supreme court seems like it's red vs blue when looked on from the outside.
I'm guessing this conference is rather left leaning, which is why they'd called that rigging, but there wasn't election fraud. It's an issue of course, since this means that rich people can essentially buy massive influence on the German democracy by clever use of social media and lies. Which may seem like the norm to a lot of people on HN, but that's not how it has traditionally been in Germany.