Urgent Fix for API response "The Quote Couldn't Be Saved"

Description

Overview:We are facing a critical issue in our Magento 2.4.3 setup related to the {{base_url}}/rest/V1/carts/mine/items API endpoint. PrivilegesOwnership...has been checked ...max persmissions on everything does not make it work
username in db has super privileges
The API is returning the following error:
"The quote couldn't be saved."
This issue is preventing the cart from being saved correctly, which is affecting the checkout process and overall cart functionality.
We need a Magento 2.4.3 expert who can quickly identify and fix the root cause of this API issue and restore full functionality to the cart-saving process.
Scope of Work:- Analyze the {{base_url}}/rest/V1/carts/mine/items API endpoint and identify why the cart (quote) cannot be saved.- Diagnose the core components involved in the cart-saving process, including the Quote Model, API interactions, session handling, and database integrity.- Check for database issues, such as missing foreign keys, table integrity problems, or corrupted records, that could prevent the cart from being saved.- Fix API issues and ensure that the cart can be updated, saved, and retrieved via the API successfully without any errors.- Test the solution to ensure that the API endpoint works correctly under all scenarios (adding items to the cart, updating cart items, removing items, etc.).
Requirements:- Deep expertise in Magento 2.4.3, particularly in cart and checkout functionality.- Strong knowledge of Magento API and specifically the {{base_url}}/rest/V1/carts/mine/items endpoint and how it interacts with the Quote Model.- Extensive experience debugging Magento’s Quote Model and API failures.- Ability to troubleshoot and identify issues with API requests, session management, database integrity, and Magento’s save operations.- Ability to deploy quick fixes to restore the cart-saving functionality immediately.- Fast turnaround with a focus on getting the cart working as quickly as possible.