It looks like the iptable_filter.o module is not being loaded.
If you do insmod iptable_filter then it should now work. This is only temporary.
You should have both ip_tables and iptable_filter modules loaded. It appears that iptables -L loads ip_tables OK but fails to load the other module.
You can make iptable_filter load up when ip_tables is loaded by doing the following:
I tried on the SL-5500. Make a new file in /etc/modutils/ called iptables.conf
In iptables.conf insert the text above ip_tables iptable_filter press carriage return at the end of line.
Run update_modules and the new text should appear in /etc/modules.conf.
I ran depmod -a but I don't think it is needed.
now iptables should work, at least the error should go away. You may need to do rmmod ip_tables to remove the module before running iptables -L. Otherwise loading iptable_filter may not be triggered.