I don't have a Comcast account (looong way from its service area), but do have a @me.com account, and was able to set it up just now, using automatic setup.
Hoping it was a temporary glitch, if not, the workaround for @me.com / @mac.com discovered before is to go into this account's setup (in AquaMail), and change login name from "user@me.com" to just "user".