Two Identical Setups, One Works, the Other Stopped!

[aprsca]

I feel like I should be able to figure this out but...

I have two identical setups running HE tunnels.  Both are running through DD-WRT routers.  Before you panick, start laughing, start chiding, etc., know that I have had these working for years!    Indeed, one of the setups still works like a charm, with uninterrupted IPv6 access.  However, recently, my other tunnel stopped working properly.  Here are the specifics on the tunnel that is not working:

1. It is set up IDENTICAL to the tunnel that works.
2. From a computer behind the router, I can ping the IPv6 address of the router.
3. From a computer behind the router, I can ping both the Client IPv6 Address and the Server IPv6 Address of the tunnel.
4. I cannot ping any other IPv6 address.  I get "Destination unreachable: No route".

With these being identical setups, routers, firmwares, and operating systems (CentOS Linux) on the computers doing the pinging, what are the possible causes for the failure of one to find a route?  If you need/want any additional information, I will be happy to oblige.  Thank you very much for helping!

[aprsca]

No, 2.6.32.  And anyway it is the same result from my android devices and Windows XP and 7 boxes behind the router as well.

An additional interesting piece of information.  From the ROUTER, I can ping any IPv6 address.  If I ping an IPv6 address from the ROUTER, then immediately go to one of the computers and ping that address, it works.... for about 10 seconds.  Then I loose the route again.

This is really strange!