Can't start AIM connection: "Messages lost the connection to AIM"

I can't bring up my AIM buddy list in Messages. Here are the symptoms (some of which may be purely coincidental) and trouble-shooting configuration variations I have gone through:

1. This is in Messages as part of Moutain Lion 10.8.

2. There is a tilde next to my AIM ID in the buddy list window. Clicking on it, reveals a message that says, "Messages lost the connection to AIM."

3. The Connection Doctor reports, "Router type: Port Restricted," with a yellow dot to its left.

4. I am using DD-WRT on my Linksys E4200, which is in beta. It has UPNP enabled.

5. Whether or not I enable the SPI firewall in my router, makes no difference.

6. I have tried switching SSL connections to the AIM server off and back on.

7. I have used these AIM servers: login.oscar.aol.com, slogin.oscar.aol.com, and api.oscar.aol.com, all with SSL on (Port 443) and off (Port 5190).

8. I have tried api.oscar.aol.com with Port 80.

At work, where I have no control over the firewall, I can log into AIM. Also, I could try to replace the Linksys with an AirPort Extreme, but it's too much work to do that right now, as a lot of stuff depends on the router being set up correctly, including our Vonage phone service.