Go to your server-manager and look down the bottom left for Report a bug and then run Configuration Report and post that here.
Your server should be in server gateway mode I think?
What is your internal network?
10.10.1.x ?
This approach I listed above worked a while back.
When? What have you changed?
I suspect the logs you want might be in /var/logs/iptables but the first place to check is messages to see if there are qny errors when you create a rule.
Note... ask these questions because we have no idea about your setup and need to understand it.