My forum members have experienced a sudden increase in this message. Like, they went from never seeing it to all the sudden a couple days ago they all see it every time they try to log in for the first time.
And then I came here to post a topic about it, and phpbb.com had me fill in a capcha because I'd exceeded the maximum allowed number of login attempts! I haven't logged in for a while.
This makes me think there's some kind of widespread phpbb attack thing going on, lots of bots trying lots of times to log in as anyone.