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I didn't know it was happening.
I own no Apple hardware, and so do not have access to a Safari browser with which I could test this.
However stats show that people using Safari on Iphone are still signing in, so it must be a config thing.
The only thing that sometimes comes up is private browsing. Mozilla Persona is used for the sign-in stuff, and that requires them to set a third party cookie. If a third party cookie is being blocked by private browser in Safari or an ad-blocker, then that is the cause.
Note: Private browsing in Firefox, Chrome and Dolphin all work fine with Persona, it's just Safari that seems to be affected.
Still doesn't work on my iPhone, still get the message in @Fintan's post. @Velocio do you know this happening?