" Warning: Permissions group is not defined: 'guest'"

What does this even mean? Every five seconds. I do not have any "guest" groups anywhere on my server. Please, stop this spam!

At default, the plugin uses SimpleConfig.yml which has a word 'guest' inside, generated defaultly.

You should update to the latest version of Autorank, as the SimpleConfig is no longer supported.

I don't even have a word "guest" in my whole configuration.

It means that you have defined a group Guest in the Autorank config, but do not have that group defined as permissions group in your permissions plugin.