The server suffered another DDoS attack on Saturday but this time I was able to identify the targeted service. It was PokerTH. For some unknown reason someone is interested in damaging the PokerTH community. Their main server has been attacked since February and some people found linuxiuvat.de and tried to continue playing Poker on this server. Apparently this made linuxiuvat a target too.
At the moment there is not much what can be done to counter this attack because filter capacity and a CDN would cost a lot of money. I hope those attacks will cease one day because controlling a botnet is not for free either.
On a more positive note, I have updated Debian's version of Minetest in jessie-backports to 0.4.14 and upgraded linuxiuvat's server as well. Have fun.