New Launcher (Invalid Session "Try Restarting your Game")

Ok well anyways, when I go to log onto Minecraft (with the new launcher) It lets me get on, but when I try joining a multiplayer server such as "Mineplex" it says "Invalid Session, Try restarting your game" so I did so. When I restarted it, it said it again. This keeps happening and I know it is the new launcher for a fact because I downloaded a Minecraft with the old launcher, and I went to log on and it worked perfectly. Any solutions?