Sorry this response is so late, but I've been having this problem too and I finally found a solution today on another forum.
If you're still having the problem, try logging into AIM from port 443 rather than port 5190.
You can do this by going to iChat in the menu bar > Preferences > Accounts, clicking on the 'Server Settings' tab, and changing the port number there. Make sure you're logged out of AIM first.
Hope that helped!