Installed the latest update of Microsoft Outlook Update 15.9 on 4/14. After installation, as soon as I launch outlook it prompts me for the username and password for the Exchange server account. Typed it in, and it asks again, etc... Never clears. Exited, went back in; same outcome. Rebooted, same outcome. Remove the Exchange account in Outlook and re-added; same problem. Removed Outlook 15.9, and reinstalled 15.8.1 and it works fine.
No sure if anyone else has run into this yet.