"Internal Server Error"

When running a webhook POST.

curl --request POST \
     --url https://api.marketcircle.net/v1/subscriptions \
     --header 'Authorization: Bearer [ACCESS_TOKEN]' \
     --header 'accept: application/json' \
     --header 'content-type: application/json' \
     --data '
{
  "event": "insert,update,trash,delete",
  "target_url": "[MAKE_DOT_COM_WEBHOOK_URL]"
}
'

I keep getting the error response:

{"errors":{"detail":"Internal Server Error"}}%

It looks like this is saying there's an error getting a response from the API, but it's vague and could mean there's an error with the target_url.

What's going on here?