For Vanguard Brokerage accounts enter the Account number and for the Document ID you MUST remove the spaces between sets of numbers.
Even with the correct credentials, Vanguard's server may return an error. My guess is that the server is too busy as I have found that sometimes the request goes through and other times it errors out.
EDIT: One other "gotcha" when entering Account number and Document ID: avoid copying and pasting the numbers.
