Comcast's routers use a 10.0.0.x IP addressing scheme, whereas every other home router I've seen uses a 192.168.x.x scheme
So if your Linksys router uses the latter scheme, that might be where the "Not a valid address" error is coming in: You may be putting in the 192.168.x.x address that the Linksys router assigned you, but the Comcast router is expecting a 10.0.0.x address.
That'd be my first guess, anyways
Why are you using two routers anyways? There's no good reason that I can think of to do so, and everyone I've seen running such a setup has had a lot of difficulties getting various things to work, especially when the two routers use different addressing schemes.
IMO I'd start with one of the two:
1) Get rid of the Linksys router and use just Comcast equipment
2) Return the Comcast modem and buy your own modem. Make sure it's JUST a modem, not a router/gateway.
Option 2 has the added bonus of saving you like $5 or $10/month, or whatever the monthly lease fee is that Comcast charges you to use their equipment.