CORS Issue Debugging in React-Flask App

I need urgent help with a CORS issue in my React app which communicates with a Flask backend. The problem is affecting session-based cookie authentication. While logging in and out works fine, I am unable to send cookies properly to the backend or make API requests.

Something I've been seeing all night: "localhost/: Access to fetch at 'http://localhost:5000/endpoint' from origin 'http://localhost:5173' has been blocked by CORS policy: Response to preflight request doesn't pass access control check: It does not have HTTP ok status."

What I need:
- A knowledgeable freelancer in CORS and session-based cookie authentication
- Someone who can pair-program with me via video call tonight
- Help in debugging and fixing the current CORS issue

My Current Setup:
- Authentication: Session-based cookies
- Flask backend: Running on Flask's built-in server
- CORS Configuration: Yes, using Flask-CORS

Ideal skills and experience:
- Proficient in React and Flask
- Strong understanding of CORS and authentication
- Previous experience with debugging CORS issues
- Good communication skills for effective pair-programming
- Available for video call tonight

I am looking forward to your bids. Thank you.