I will post my solution tomorrow or the day after tomorrow. I have to make a tutorial so that other student are able to reconstruct my whole setup, which I'm going to do this weekend.
The problems of my setup came from the combination of the sql cluster, the apache cluster and a nfs.
This problem in particuliar was caused by two lines of code in /etc/network/interfaces:
up iptables -t nat -A POSTROUTING -j MASQUERADE -s 192.168.0.0/24
down iptables -t nat -D POSTROUTING -j MASQUERADE -s 192.168.0.0/24
Thanks for the reply!