Copy/Move Attachments: Object reference error

Hello there!

We are trying to copy a file from one document-library to another within the same site. No matter what we do, it keeps giving the error "Object reference not set to an instance of an object.".
We have tried setting the URL with workflow context, with relative path and with full URL - nothing works.
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Please help asap.

Thanks in advance.

Kind regards,
Asmita