You may have a problem in the inittab file of your system. cd to /etc, enter cat inittab, if the system response is that there is no inittab file or the file is empty then you found your problem.
The following command will rebuild the inittab file on a SCO 5.0.7, I do not have SCO 6, you may have to verify that the procedure is the same before you do the following command:
cat /etc/conf/cf.d/init.base /etc/conf/init.d > /etc/inittab
Another possible answer has to do with the files associated with the WHO command. If these files have the incorrect permissions you will not be able to login.
This files are located under /etc and their names are utmp, utmpx, wtmp, wtmpx.
the proper permissions are: 0777, to change the permission you would enter chmod 0777 file-name.
If this does not fixes the problem, then you may have to go to the actual location of the file, (The files under /etc are links)
The location of the linked files in SCO 5.0.7 is /var/opt/K/SCO/Unix/5.0.7Hw/etc
The permissions for the linked files is 0664
Hope this will help you
Joe