Originally Posted by
verso2
Today, I tried to login to my MileagePlus account on United.com, using that alphanumeric username, and got this response: "Please enter only letters A-Z or numbers 1-9." Now, there's a long MP account number associated with the alphanumeric username I've been using, so I entered that instead, along with the same password. It worked.
When you say Alphanumeric username, do you mean something like AB123456 or ABC12345? And you when say "long MP account number" do you mean something like 03210123456?
If so, you've got your terms wrong. ABC12345 is your Mileage Plus number. It's not a username, it's the MP 'number' assigned to your account.
03210123456 *was* your United Mileage Plus number up until they merged the United and Continental systems and moved over to the Continental-style numbers as above above 8 years ago.
To login to the website (and app, and everywhere else) you should be able to use your MP number (ie, ABC12345). Interestingly, using your old MP number does indeed still seem to work on the website, but that's almost certainly just for legacy reasons and not really what they intend you to go given those numbers aren't used for anything any more.
If your new MP number didn't work, then either the website had a glitch, or you mistyped your username or password.