FileMaker just released version 2 of 13. Changes/bug fixes for both "Insert from URL" and XML are mentioned. Curious/hopeful that this solves your problem. Let us know!
This is the first time I've heard of error 802 associated with Insert From URL, although I'm not sure the script step is the problem here, more likely it's something to do with either the permissions from the user device connecting to the server, or an issue with the file on the server itself.
Here's something about the error, but it's for FMS12, not sure if any of the steps would help you.
It also has been linked to permissions errors and file reference errors:
Thank you everyone. The v2 Updater for FMP 13 fixed our problem!