For users who can access certain web sites, but have trouble logging in. Usually this is caused by extensions. Yahoo Toolbar is know to cause problems logging into Yahoo mail. David emailed me something about the Skype causing log in problems at <http://mdh.blackboard.com/webct/entryPageIns.dowebct>, which could be worked around by Ctrl/Cmd+clicking on the login button.
From Bug 475381, the article in the URL is for being unable to log into sites in general. Marking fixed, though this needs a review before moving to the KB.