ubuntu 12.04 LTS on a Dell vostro 1520. Recently switched to nouveau display driver since the NVIDIA driver was causing serious problems (see thread #2107969). Ever since the system can't be unlocked after the lid was closed. When I reopen the lid I have a black screen, a moving mouse cursor, the time of the lid closed top middle and a lock with the user name next to it top right. There is no way to unlock the system, the screen stays black no matter what key you press. With the NVIDIA driver a login screen used to pop up after the mouse was moved or a key was pressed.
How can I diagnose the error and possibly fix it?