Just asked a question but before I could do that, I needed to login here… which was not easy! I kept being told there was a problem with my credentials—and I know my password and it is saved in Safari. Changing the password (in Safari) doesn't fix it: you can change it successfully, but then you are sent back to the same login screen.
Two things "fixed" it: logging in on Chrome, to check it worked—it did—and quitting Safari, and then logging in.
I am using the Sonoma Beta, and Using Safari Version 7.1.1 (Build 701.49349.100) so it is possible this is beta-related. But, I am interested to know whether Quicken just doesn't like Safari (that is, if its web programmers assume everyone uses Chrome)?