Thanks to Comcast rep Carlo (and the forum user who posted it) for pointing out this partial solution if you can't get any of the links to work on the Inbox Preview page. Here is a URL you can PASTE into the address bar to load your full message list so you can read and send mail, check voicemails, get to Preferences, etc. Paste this into the address bar when on the Inbox Preview page. Don't click any links or tabs!
Wow, thanks for the replacement link! It works great. My Comcast "help" person didn't even know there was a problem -- her supervisor told her they knew about it and were working on it. But they've offered no help so far. (Don't you think "Help" -- yes this is meant as ironic -- desks would have morning meetings or provide daily written updates on known problems to the customer service folks? Sounds like they need a quality manual, hmmm?)