Most of the issues I found are related to new installs, root password being disabled by default, or things falling apart after some changes. None of that applies here: in my case nothing was changed, nothing new installed; everything was working before just fine.
Tried: su
, su -
, su root
, su - root
- all return su: Authentication failure. Even changed the password to the same one using the VPS control panel, nothing.
This happens on a Debian 10 ('Buster') VPS powered by Virtualizor.