In German law, any contract, including bets, is void if it is “sittenwidrig”. For example, if your wager in a bet is to become one’ slave in case you lose, this bet is void.
I suppose they never specified the wager. But, given the topic is prediction markets and the topics of the bets, I thought it was implied that the wagers would be exclusively in money. With that in mind, is there any example of a law against specific topics being betted on?