I haven't played this game since beta, until a couple of weeks ago, so this is new to me, but I heard at one point that this has happened before and apparently, it's the fault of the scheduled maintanance from last night.
Just logged on today, then when I switched channels, I got the alert.
I was all like "lolwut?" 'cuz I hadn't even run a dungeon yet at the time. But I did my dailies at Bohemoth and helped a friend with some of his quests, then logged out and the next time I logged on, I got this.
Apparently, there's already a thread on the Nexon forums that's already 9 pages long, consisting of posts from other players who've been hit with the banhammer for no reason other than logging on today.
Nexon Forums - Post here if you got Abnormal Gameplay after Maintenance