Hi. It could be two things...
If root cannot log in - something got messed up in your /etc/passwd file and the login shell was spelled incorrectly (Last part of the root line). Make sure that it reads /bin/bash
If root can log in - something is wrong with one of your login scripts. Depending on how many scripts you have it could take some time to find. I would start by searching your /root/.bash_profile and your /root/.bashrc files for /bibn/bash. If the problem isn't in those, check what scripts they are running.... if it's not THOSE.... then check /etc/bashrc.....
tip #1 - easy way to scan a file is to use the command:
cat /etc/bashrc | grep bibn
Disclaimer - I'm not sure where slack keeps the files mentioned above... my experience comes from other distro's.