What exactly triggers jQuery ajax success?

The success handler will be triggered if

• The response has HTTP status code 200 (yours does)
• jQuery is able to deserialize the response according to the Content-Type header in the response (or the dataType option, if you provide it, which overrides the Content-Type header)

So for instance, if your response had a Content-Type header of application/json or application/xml, the response you've quoted won't trigger the success handler because it cannot be successfully deserialized as either JSON or XML.

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Your latest edit (as of this writing) reveals the problem:

Here are the response headers from jQuery getAllResponseHeaders()

...

Content-Type: text/xml; charset=utf-8

I suspect your response isn't valid XML, so jQuery can't deserialize it as XML, so it's failing.