I'm trying to post a story on my instagram page using the API.

URL: POST {USER_ID}/media?image_url={URL}&media_type=STORIES&user_tags=%5B%7Busername%3A%27therock%27%7D%5D

The returned error is "message": "An unknown error has occurred." without any detail. When I remove the "user_tags" parameter it works without problem. What's the problem with the user_tags parameter? The error persists even when I create exactly like the documentation says.