Now people who've been affected by this are saying that it went away on its own after about a week. So I dunno, if this would really hugely inconvenience you try to not log in on additional devices/browsers until the 4.2 update that's supposed to hopefully resolve this for good.
For a long time now, D1P has been limited to three logins at a time; logging in on a fourth device or browser would log you out everywhere else. Unfortunately, multiple people have been reporting that they've started experiencing being limited to ONE login at a time.
The good news is, Invision Power Services (the company that makes our forum software) is aware of the issue and will be addressing it in version 4.2 of the software, which is the next big update. The bad news is, they announced about a month ago (the beginning of March 2017) that the update will be coming out in "mid 2017", so we probably have at least another couple of months to go before this is resolved.
In the meantime, I apologize to those affected for the inconvenience, and would suggest to everyone else to not log in to additional devices until this is resolved if this is something you don't want to have to have to deal with. I'm still not 100% sure on why it's not affecting everyone and why it didn't hit everyone affected at the same time, but the timing of when the reports of this started here mostly lines up with when I've seen reports of other sites having this issue starting, and I suspect that the problem is trickling in because of people happening to hit a fourth login that logs them out everywhere else, and then proceeding to be limited to one login at a time after that.