"đã đăng nhập" is listed by Duolingo as "entered". Is that translation specific to "entering" an account, profile, social media, etc - or can it also be used for physically entering a building? IE a synonym "đi vào"?
This should be "into", not "in to", in English.
"Who signed in ON my account" should be accepted, and actually sounds more natural than the "correct" answer.