This server is issuing offline-mode identities
What causes it
The UUIDs handed out at login are version 3, generated from the username instead of issued by Mojang. That means online-mode=false, or a proxy in front that is not forwarding real identities.
How to fix it
Two consequences, both worse than they look. Anyone can join under any name, including your admins, because nothing checks the account. And the UUID is the player identity: flip online-mode back to true and every inventory, home, claim and permission tied to the old UUID looks lost, because the player is now a different person to the server. Set online-mode=true in server.properties before the world has history, or configure proper forwarding if a proxy is meant to be handling authentication.
Not sure this is your problem? Paste your log and Roastd will tell you which of these it actually matched, and what else it found.
Check your own log