After a login attempt with correct credentials the login page is displayed again with no error message shown or one of the errors.  Most likely this is an error related to session creation problems.

• Unknown combination of login and password or insufficient permissions.
• Your session was closed due to security reasons.

The session data is not writeable by the web server.

All-

See the php.ini for the directive 'session.save_path'.

• Locate this directory
          
• if this directory contains no files, it most likely is not writeable to the web server
• check if the directory is writeable for the web server
• check if it spelled correctly (e.g. /session/ instead of /sess/)
• Check if the disk containing this directory is full.