Running the command su - <username> and receiving error "Cannot set process environment"
The problem is with permissions along the login procedure. A file or directory, which has to be accessed during the login, does not have "Read" set for the users.
The following files are part of the login process:
The root directory should be checked for correct permissions. Generally it will need 755 permissions ("chmod a+rx /") to allow users other than root to login to the system.
This error can also indicate that the users shell is not correct.