If a Contribution page has a profile that creates a Drupal User, then the user gets a message after submitting the form that 'email is required' and no Drupal user is created.
Not sure if this is only affecting Drupal.
Looks to me like to me like this was a problem in 4.7.20 and 4.7.21 and am picking this JIRA issue is involved and have created this JIRA issue for the fix.