вот:
В файле BlockGuard замени
if($block->getId() == Block::CHESTand $this->isFlagDenied($block, ‘chest’, $player) )
на
if( ($block->getId() == Block::CHEST OR $block->getId() == Block::SHULKER_BOX) and $this->isFlagDenied($block, ‘chest’, $player) )