20 Players insta crash!

causing conflics with Death Messages: The Death Cause is 'PvP Melee Attack' but the server can't get the assasins name! This is caused when other plugin tries to use NPCs/Mobs or Commands as Players.

Can someone explain this sentence?

Plugins:



And right when i just checking i saw that sentence and alot of players crashed.

Is someone able to help me with this?