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During the Auth & Auth flow on Sandbox, after the user signs in, instead of navigating to the AcceptURL page, I am getting the page cannot be found error on the following url:
Why is this happening? Is the Sandbox down?
The easiest way to check if there is a problem with Auth & Auth on Sandbox, is to try using the Authentication Token Tool. If that works fine, then it is most likely that there is an issue with your AcceptURL. Try navigating directly to your Accept URL and see if it works as expected. You may also want to try using a tool to monitor the http traffic and see what is actually being sent and received. If you continue to have issues, you can use Developer Support and we can help you diagnose the issue.