Today I tried to boot Ubuntu with 9wm (which I had installed before), first by modifying the GRUB boot from 'quiet splash' to 'text', later adding it to LightDM. As it didn't work, I removed the LightDM entry and restored GRUB. But when I try to login with any window manager in LightDM, it just stops and opens LightDM again. If I try to run startx from a TTY, it shows errors, saying all time that No protocol specified. What can I do?

UPDATE: It just happens with my user, logging in with another one works.