The Mac OS may cache an Identity Preference for the sites you attempt to access. In some cases this can be problematic in that it is attempting to use the wrong identity when authenticating. You have the ability to remove these preferences so that the next time you log into a website, no identity is assumed. Here are the steps in which you can remove Identity Preferences from your Keychain:
First, launch Keychain Access which is located in your Applications > Utilities folder
Next select Login under Keychain, then All Items under Category
In the search bar, type part of the web address for the site you are trying to access. This will filter out anything not related to the site. If you see any Identity Preferences for this site, select the preferences and delete them by pressing the delete key on your keyboard. You should be prompted to enter your Mac password in order to remove them. Once they are deleted, quit out of Keychain access, Quit and reopen Safari and try to log into the site.