When I open a world in 15w43b for the second time, then I can't do anything in it

When I open a world in 15w43b, then everything works as it usually does and is supposed to. Whenever I leave and come back, however, it acts as though I am in spectator mode, but my hotbar is shown and I can still open my inventory. Whenever I attempt to use commands, I can type them into the text box and press enter, but nothing happens. I am then stuck with that forever and can't do anything. Please fix this!