I am not bothered with this new login thing except for some reason when I open up the launcher it stays for 3 seconds long enough for me to see "Launcher updating" and then it shuts off and a message box turns up saying "only one application can be open at a time" this has been going on ALL DAY!!! Pray tell what the HELL is going on??
Check your Task Manager under the Processes tab to make sure there isn't an errant version of the application stuck in the background somewhere. If you see it listed, force-close it and you should be up and running.