As a normal user do "grep '^root:' /etc/passwd". Don't post the
result here. Does it end in /bin/bash or /usr/sbin/nologin
or something else?

Also if you haven't tried "sudo bash" please try that as it

is only the same as "sudo -i" if root's login shell is bash.

Which it is, /bin/bash.

Next?

Thanks Mike, Gene