Hi, Thomas.
Despite we've recommended an upgrade to 2.4.4 for you instance, I'm not sure whether it will resolve this issue. It seems more like peculiarities of Safari to me. If you don't mind to dig around a bit, I could propose to capture HTTP requests sent both by browsers that work, and the Safari browser, during the login flow, and compare them, trying to find out what differences are, and whether they conform to some standards.
Feel free to share your findings with us, we could think together what to do with it then.
Best regards,
Alex.