This is almost always a response to an invalidly formatted json string. I’d start from scratch with the sample body that documentation provides and then work forward from there.
EDIT: Also found out it returns 500 if I filter by email for an address that’s not in our system. Should really return a 404 or empty result instead of an internal server error.
If you’re just getting a list then it can also be a performance error (which would be a 5xx error). have you tried setting a limit of 10 or something lower to check this?
That didn’t fix it either. I didn’t expect it would as the original problem is for records with zero emails sent (so the filter wouldn’t be filtering anything). Also the request returns very quickly. Chrome tools say 164ms.