I went round with this for an hour: trying, submitting, calling the bank, calling intuit, checking and validating. I knew the number was correct. Then it occurred to me that the number being imported into the form is from a different character set (web dev thing) and character sets can look the same but aren't the same so validity checks break. So I selected and copied the number (same number) from the top - web page - box and pasted it into the form box below. It worked!
Hope that helps someone
