Safari now defaults a privacy setting to "Prevent cross-site tracking" which is required to use the Online Planner embedded on a website. It may cause the browser to prompt you with the login dialog again.

You can turn this privacy setting off by going to the Safari Preferences > Privacy > check OFF "Prevent cross-site tracking" OR you can launch the embedded planner into full screen mode by selecting the full screen on the bottom left of the Planner (must accept the cookies prompt).