info about the server:
eth0: gw internal Zone in Firewall
eth1: 193.x.x.100/24 gw 193.x.x.254 external Zone in Firewall with ssh, http, https allowed
default route:

Everything works fine, until the firewall is started. Then only connections to eth0 are possible.
If i change the default route to 193.x.x.254 only connections to eth1 (ping, ssh, http, https) are possible, but eth0 is completly down.

I have to disable the firewall and reboot the whole server to get both NICs to work again.
Even if i change eth1 to internal Zone it does not work.

I had the same problem without the firewall. This was solved by:
net.ipv4.conf.all.rp_filter = 0
in /etc/sysctl.conf