Thanks to all for advice/explanations/help.

No, the 8-char MP account id did not work for several days (on the website; the United app on iPhone was OK).

I finally had to call United, but 1) did not have a huge phone-tree problem in getting to talk to a rep; and 2) the guy I talked to was able to reset my account, which then started working correctly again.

So: problem solved, and keep the rep/reset option in mind if you have sudden, odd problem in logging in.