I have this message all time on my chat => death.attack.mfr.grinder

And on server => [Server thread/INFO] : death.attack.mfr.grinder

MFR version : 2.8.0RC7.75

Resolved by 44b5e71

Try release MineFactoryReloaded-[1.7.10]2.8.0RC7-71.jar or last build.

COFH Core was updated and created new config files. Look for the config file in your config directory: config/cofh/core/common.cfg.

Make sure the setting is set to false in the "general" section up top.
"Set to true to display death messages for any named entity.
B:EnableGenericDeathMessage=false"

This is not a Minefactory reloaded issue.

The fact that it outputs "death.attack.mfr.grinder" instead of a localized message is

Most likely this.