A quick update on the situation
I have been using BotCheck to fight comment spam on this site. After a while I realized that BotCheck also killed all trackbacks. So, I deactivated BotCheck and installed “Bad Behavior”.
Still, after a while some spam got through. To prevent that I installed SpamKarma 2. After I installed SpamKarma I have had absolutely no spam, and one single case of a trackback that was stopped unintentionally.