I meet this problem, too.
I downloaded Opera on AppStore and then the first time when I trying to login my account, the same tips occured, really annoyed, can't login my account means I can't sync my data any time I want.
Hope this issue can be fixed soon, or, I may consider switch to other broswer.
Best posts made by Tony997
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RE: Connection Error: cannot sync dataOpera Mini
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RE: Unable to sign in to OperaOpera Mini
The same to me, I use opera mini 16.0.12 on my iPhoneX, and the ios version is 11.4.1