After I turned off my server or reload it, the guards are all invisible? Creepy..

Are you using LibDisguises? There's a bug with that plugin that I can't figure out.

Im not using LibDisguises but Im using a free minecraft hosting service, the bugs seems to disappear if I reload my server for a several times. But they deleted my server (with tons of works I have done to it) a while ago because I forgot to renew it, Quite sad thought.