If it is the case that the issue is because of being logged in on another computer (not sure yet, need to test a bit more), I would suggest that you get logged off automatically on the other device or at least make the error message something meaningful (something a bit more specific than "Login failed! Please login again").
Ok, I'm having the same issue, but only on one (my new) computer. On the old one, I'm still able to log in. (Also: I seem to have an xbox profile ... frankly though, the MS/Mojang/Xbox user/profile jungle is a mess 😛)
If it is the case that the issue is because of being logged in on another computer (not sure yet, need to test a bit more), I would suggest that you get logged off automatically on the other device or at least make the error message something meaningful (something a bit more specific than "Login failed! Please login again").