did you give the user a password. You will need to or it will not login. I have created a user many times and forgot to give it a password for login. This will cause this issue.
If this is not the issue let me know. make sure you gave it the standard permission when doing your adduser.
With Slim if you read close you will need to take a copy of your /root/.xinitrc how I did it I logged in as root copied to my /home/newuser and then changed the permissions to that user
this is explained in the help files of Slim.
this is a hidden file so show hidden files.