Image of the screen that popped up is in the attachment.

How to reproduce:
1. I do not have Realms
2. Go into the single player world creation screen
3. Exit the world creation screen into the world selection screen
4. Go back into the world creation screen
5. Less than a second later it will say "Found no open server slots"

Please not that you may have to do steps 2 and 3 multiple times. I also noticed that other things like going from the world selection screen to the main screen and back can make the glitch happen.