Debug an error message for Finance & Banking

You are an expert in this kind of work, and you know the Finance & Banking sector from the inside.

## Your task
Help me debug this error in a system at a bank offering retail and business accounts. Explain what the error means, list the most likely causes in order of probability, give a step to confirm each one, and the fix for each.

## About my situation
- Who this is for: retail customers, analysts and compliance officers
- What we must respect: KYC and AML regulation, audit trails and data security
- How we measure success: customer churn and cost-to-income ratio

## What matters in Finance & Banking
- Rules and standards that usually apply: KYC and AML/CFT rules; central-bank regulation (RBI in India, the Fed and OCC in the US, the FCA in the UK); fair-lending and consumer-protection rules; PCI DSS for card data; required risk disclosures; never promise guaranteed returns.
- Common risks to plan for: fraud and money laundering; loan defaults; regulatory penalties; cyberattacks; mis-selling complaints.
- Tools commonly used: core banking system; CRM; loan origination system; mobile banking app; BI dashboards and Excel.
- Words people in this field use: KYC; AML; NPA; CASA; NIM; credit score; EMI; collateral; risk appetite.
- Rules differ by country. If I have not said where I work, ask me, and do not present one country's law as if it applied everywhere.

## What I will give you
Error and stack trace:
[PASTE ERROR]
Relevant code:
[PASTE CODE]
What I already tried: [WHAT YOU TRIED]

## How to work through it
1. Explain the error message in plain words.
2. List possible causes from most to least likely, given the code and what I already tried.
3. For each cause, give one quick check to confirm or rule it out.
4. Give the fix for each cause.
5. Suggest how to stop this kind of error from coming back.

## What to deliver
- Error explanation
- Ranked causes with a check for each
- Fixes with code
- Prevention tips

## Rules
- Make every point specific to my situation and my industry; replace generic advice with detail that fits.
- Use plain language, short sentences, and tables where they make the answer easier to scan.
- Use only the facts I give you. Do not invent figures, laws, sources or quotes. Mark anything you had to assume with "Assumption:".
- If something important is missing, ask me up to 3 short questions before you start.

## Before you answer
Check your draft against this list and fix anything that fails:
- The code works as written: no invented libraries, functions or APIs, and versions are named where they matter.
- It is written for a bank offering retail and business accounts; nothing in it would read the same for any other business.
- Nothing in it breaks KYC and AML regulation, audit trails and data security.
- Every item under "What to deliver" is included.