@Plural USD Apologies for the delay in response and all the inconvenience caused because of the issue!!!
As mentioned here the mentioned error is because One of the specified Azure Blob(s) is not found.
Can you please confirm if you have already followed this article to use custom activity.
Also I came across one similar old issue on Stack Overflow here where configuring the SAS resolved the issue.
Some more information can be found here as well.
Can you please check and confirm if above works for you or not.
Hope it helps!!!
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@Plural USD Any update on the issue?
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