I've got a Pi book. Since last time I logged in, a new thing appears which asks me to enter username and password. It's called default xsession. This was already weird because it didn't happen before. If I try to login as Pi, blackscreen appears with a square colour (little rainbow in the corner) and I am in the login screen again. If I login as root everything works.
Not enough reputation to comment, but the little rainbow in the corner is the pi telling you it doesn't have enough power. Check / change your power supply. Not sure if that will solve the problem, but it will at least avoid another one.
See here for information on the warnings.