Here are some potential reasons and solutions:

1. Mixed Content

Ensure that all elements (scripts, images, iframes, etc.) on the page are being loaded over HTTPS, not HTTP. Mixed content (where some resources are loaded over HTTP while the main page is HTTPS) can cause the browser to flag the page as insecure.

2. HTTP to HTTPS Redirection

Verify that the redirection URL in your Odoo controller explicitly uses HTTPS. If the controller is redirecting to an HTTP URL, the browser will warn that the connection is not secure.

3. Payment Gateway Configuration

Double-check the configuration of your payment gateway. Ensure that the payment gateway callback URL is set to use HTTPS. Some payment gateways may default to HTTP if not explicitly configured.

4. Correct SSL Certificate

Although you mentioned that your SSL is working fine, ensure that the SSL certificate is correctly configured and covers all subdomains or alternative domains that might be involved in the redirection.

5. Content Security Policy (CSP)

Implement or review your Content Security Policy headers to ensure they are correctly configured to enforce secure content delivery. This can help prevent any non-secure elements from being loaded on the page.

6. Custom Redirects in Controller

Review the code in your Odoo controller that handles the redirection. Ensure that it is using a secure URL (HTTPS) when performing the redirection after the payment is processed.

7. Browser Cache and Cookies

Sometimes, old cached versions of pages or cookies might cause this issue. Try clearing the browser cache and cookies to see if the issue persists.

8. Update Odoo and Payment Modules

Ensure that your Odoo instance and payment modules are up-to-date. Sometimes, security-related updates are included in newer versions that could resolve this issue.