I use the standard launcher, there is NO second session open. I have to close the game TWICE and relaunch to MAYBE get into a server (standard, vanilla servers), but if I leave the server with the game open, and immediately go back into it, it gives that error as well. Without me having closed the app.
This only began happening since the 1.8.1 update, and every time I open the launcher, I have to sign in again for some reason. I can only sign in with my email, because my username (I changed it a few days ago) is considered "invalid" if I attempt to log in with it. Although, when I log in with my email, my new username is the one that ends up being used when I play.
What's going on? I checked Task Manager (I use Windows), and there are no other Minecraft or Java's open. I'm tired of having to relaunch repeatedly, AND logging in constantly for no reason. I have uninstalled and re-installed Minecraft twice since this began happening.