▼テンプレート
[EC-CUBE] 4.0.3
[レンタルサーバ] X server
[OS] Linux
[PHP] 7.3.31
[データベース] MySQL5.7
[WEBサーバ] Apache 2.4.6
[ブラウザ] Google Chrome
[導入プラグインの有無] 商品オプションプラグイン、まとめ買い価格設定プラグイン、会員項目追加プラグイン、マイページ/ファイルアップロードプラグイン
[現象]
商品購入で「注文する」をクリックするとシステムエラーが発生します。
解決方法が分からず困っています。
原因が分かりましたら、ご教授のほど何卒よろしくお願いします。

デバッグモードでのエラーログは下記のような状態です。

A new entity was found through the relationship 'Eccube\Entity\CartItem#Cart' that was not configured to cascade persist operations for entity: Eccube\Entity\Cart@0000000006a5daba000000005d8be7b0. To solve this issue: Either explicitly call EntityManager#persist() on this unknown entity or configure cascade persist this association in the mapping for example @ManyToOne(..,cascade={"persist"}). If you cannot find out which entity causes the problem implement 'Eccube\Entity\Cart#__toString()' to get a clue.

【ERROR】

front Doctrine\ORM\ORMInvalidArgumentException: A new entity was found through the relationship 'Eccube\Entity\CartItem#Cart' that was not configured to cascade persist operations for entity: Eccube\Entity\Cart@000000001bdcbcf9000000000ed1a550. To solve this issue: Either explicitly call EntityManager#persist() on this unknown entity or configure cascade persist this association in the mapping for example @ManyToOne(..,cascade={"persist"}). If you cannot find out which entity causes the problem implement 'Eccube\Entity\Cart#__toString()' to get a clue. (uncaught exception) at public_html/vendor/doctrine/orm/lib/Doctrine/ORM/ORMInvalidArgumentException.php line 102

【CRITICAL】

request Uncaught PHP Exception Doctrine\ORM\ORMInvalidArgumentException: "A new entity was found through the relationship 'Eccube\Entity\CartItem#Cart' that was not configured to cascade persist operations for entity: Eccube\Entity\Cart@000000001bdcbcf9000000000ed1a550. To solve this issue: Either explicitly call EntityManager#persist() on this unknown entity or configure cascade persist this association in the mapping for example @ManyToOne(..,cascade={"persist"}). If you cannot find out which entity causes the problem implement 'Eccube\Entity\Cart#__toString()' to get a clue." at public_html/vendor/doctrine/orm/lib/Doctrine/ORM/ORMInvalidArgumentException.php line 102