Mc version - 1.16.5
oldcombatmechanics version - v1.10.0

Just updated the plugin to v1.10.0 and latest when I went to see the console for something else I saw this spam in the console from a while.
https://pastebin.com/AK3j7Dhn

Do tell me if I have to downgrade the version back or have to do anything else.

This seems to do with the Spartan anticheat check - are you using that plugin? Can you post your config.yml please?