When trying to change a user’s shell via WHM, you may occasionally see the following error:

Shell could not be modified

As shown below:

Attempting to change the shell via command line fails as well:

# chsh -s /bin/bash myuser
Changing shell for myuser.
setpwnam: File exists
Shell *NOT* changed.  Try again later.

This means there is a lock on the /etc/passwd file.  Simply deleting /etc/ptmp fixes this problem:

rm /etc/ptmp