I've gotten it too, although I've only ever played Closers at my house. Restarting my computer usually fixes it, but if that's the reason you're getting the pop-up, I wonder why I get it sometimes. @_@
Edit: Actually, logging out and logging back in on the actual Closers page let me start the game just fine.
I also got this problem, well I did refresh the page while the patcher is patching to check if it did log me out and the results is I am still log on the page.. but when I start the game I get this error.
I even tried to relog before clicking that yellow button no luck.
edit: lol I can play kritika and even elsword only this game derping...
still no idea guys? saw a guy in fb said he found a game agent fix from a forum.. not sure if this is the forum he meant.
Well if by any chance I got any pop up, I'll just log out and log in again (in my browser), It's just because I don't understand what is that. And also there are some cases like your connection, or some error when you load the page of Closers and that's why it will affect Closers game/client
hmmm, so prolly the connection of that place is the one causing it? oh well..