Velocity and the backend do not share the same secret
Applies to Paper, Velocity.
What it looks like in your log
Unable to verify player details
What causes it
Modern forwarding is on, but the forwarding secret in Velocity does not match the one in the backend paper-global.yml, so every join is rejected.
How to fix it
Copy the contents of Velocity’s forwarding.secret file into proxies.velocity.secret in config/paper-global.yml, confirm velocity.enabled is true, and restart both sides.
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