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The signin flow does not redirect to my site, instead it returns a Page Not Found error
There is most likely a problem with your firewall or SSL setup. The most common symptom of this is that you go through the signin flow, and after clicking the Agree and Continue button on the Consent Page, you are directed to https://www.ebay.com/n/error with a Page Not Found error.
1. Check your server settings to ensure the problem is not on your end.
2. Call GetReturnURL to verify that your accept url is correct.
3. Try setting your accept url ( via SetReturnURL in update mode ) to a public site like https://www.google.com. If eBay is able to redirect to that site, this indicates that the problem is on your end.