I had this issue a couple of years back, but I forget what solved it. I think the apartment I was in switched to a different ISP for unrelated reasons (viz., the old one was bad) and that cleared it up.
A failed or successful (in the sense of "right password") attempt by a hacker on a different IP would generate a one-time password message, that's part of the point of using them. However, unless the hacker has uncannily good timing, those messages wouldn't match the timing of the real user's login attempts.
So if you see a one-time password sent and you're sure you didn't attempt to log in, it's probably an attempted intrusion. But if you were actually logging in, it's probably a local- or ISP-level issue.
I have the same problem. I have Chrome clear all of my cookies when I exit the browser. Whenever you log in from a "new" browser (aka no stored cookies), the login service asks for an extra security step.
EDIT: ooooh, you are getting one-time password requests from the launcher. Do you play on multiple computers? or allow someone from a different IP address to log in to your account? I think they base the one-time password request on IP's.
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