@Mark Anderson I think the most likely scenarios that this error could occur is because your URL of file is not made public for the service to read the file which is most likely due to permission settings on your container or blob or if the file is available to read it could be password-locked, you must remove the lock before submission.
Could you check if any of the same is applicable?
Also, From the code perspective, I would check if any publicly available form or document can be used with the same code, for example you can use the URL mentioned in the FR documentation.
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