What is your server version
 

The plugin (PerWorldInventories) on PlayerServers is more that 1 year old and its very bugy! it deletes invenotry if switching gamemodes and sometimes even deletes diferent world inventories  if you swich betwen them. You still have 2.2.0 version an there alredy exists 2.3.1! So please update this plugin alredy, i'm getting realy sick of randomly deleted inventories. And have a nice day.