This is a turbotax bug.
The restrictions in TT software are incorrect.
Play around with typing different things in the login box, and you will find that a shorter login with variations of numbers or letters are OK.
Member rajdsing said, "The solution is to login to Schwab, navigate to services > Security Center > Enable Third Party Access".
That might be a solution for some who have not allowed such access, but it is not the solution to this specific problem. As I mentioned two weeks ago, I had already (years ago) enabled such access, yet still experienced the problem.
Maybe it has been fixed by now. I suspect that nobody from TT would ever have the decency to mention that here or give an explanation. I suppose I could check to see if the problem persists, but I would not like to risk tampering with my return. I'll check next year (unless some other service becomes a better option...)!