【追記】
管理画面のエラーログ該当箇所には以下のように表示されていました。


admin.ERROR [935c910c] [f6bec60] [1] [Eccube\Log\Logger:log:64] - システムエラーが発生しました。 ["Neither the property \"RelatedProducts\" nor one of the methods \"getRelatedProducts()\", \"relatedProducts()\", \"isRelatedProducts()\", \"hasRelatedProducts()\", \"__get()\" exist and have public access in class \"Eccube\\Entity\\Product\".","/home/***/********e.net/public_html/shop.********e.net/vendor/symfony/property-access/PropertyAccessor.php",429,"#0 /home/***/********e.net/public_html/shop.********e.net/vendor/symfony/property-access/PropertyAccessor.php(93): Symfony\\Component\\PropertyAccess\\PropertyAccessor->readProperty(Array, 'RelatedProducts', false)\n#1 /home/***/********e.net/public_html/shop.********e.net/vendor/symfony/form/Extension/Core/DataMapper/PropertyPathMapper.php(97): Symfony\\Component\\PropertyAccess\\PropertyAccessor->getValue(Object(Eccube\\Entity\\Product), Object(Symfony\\Component\\PropertyAccess\\PropertyPath))\n#2 /home/***/********e.net/public_html/shop.********e.net/vendor/symfony/form/Extension/Core/DataMapper/PropertyPathMapper.php(51): Symfony\\Component\\Form\\Extension\\Core\\DataMapper\\PropertyPathMapper->getPropertyValue(Object(Eccube\\Entity\\Product), Object(Symfony\\Component\\PropertyAccess\\PropertyPath))\n#3 /home/***/********e.net/public_html/shop.********e.net/vendor/symfony/form/Form.php(378): Symfony\\Component\\Form\\Extension\\Core\\DataMapper\\PropertyPathMapper->mapDataToForms(Object(Eccube\\Entity\\Product), Object(RecursiveIteratorIterator))\n#4 /home/***/********e.net/public_html/shop.********e.net/vendor/symfony/form/Form.php(482): Symfony\\Component\\Form\\Form->setData(Object(Eccube\\Entity\\Product))\n#5 /home/***/********e.net/public_html/shop.********e.net/vendor/symfony/form/FormBuilder.php(206): Symfony\\Component\\Form\\Form->initialize()\n#6 /home/***/********e.net/public_html/shop.********e.net/src/Eccube/Controller/Admin/Product/ProductController.php(419): Symfony\\Component\\Form\\FormBuilder->getForm()\n#7 /home/***/********e.net/public_html/shop.********e.net/vendor/symfony/http-kernel/HttpKernel.php(158): Eccube\\Controller\\Admin\\Product\\ProductController->edit(Object(Symfony\\Component\\HttpFoundation\\Request), NULL, Object(Symfony\\Bundle\\FrameworkBundle\\Routing\\Router), Object(Eccube\\Util\\CacheUtil))\n#8 /home/***/********e.net/public_html/shop.********e.net/vendor/symfony/http-kernel/HttpKernel.php(80): Symfony\\Component\\HttpKernel\\HttpKernel->handleRaw(Object(Symfony\\Component\\HttpFoundation\\Request), 1)\n#9 /home/***/********e.net/public_html/shop.********e.net/vendor/symfony/http-kernel/Kernel.php(201): Symfony\\Component\\HttpKernel\\HttpKernel->handle(Object(Symfony\\Component\\HttpFoundation\\Request), 1, true)\n#10 /home/***/********e.net/public_html/shop.********e.net/index.php(76): Symfony\\Component\\HttpKernel\\Kernel->handle(Object(Symfony\\Component\\HttpFoundation\\Request))\n#11 {main}"]