Dynamic Field Values won't load

I am working on a custom email integration that will send mail using a user-specified template. I need to build an automation that lets the user select a mail body template:

However, my issue is the list of templates never loads from my URL. If I hit the URL in postman, I get a result like this:

[
    {
        "id": 1,
        "title": "Test",
        "outboundType": "text",
        "inboundTypes": [
            "text"
        ]
    },
    {
        "id": 2,
        "title": "Test 2",
        "outboundType": "text",
        "inboundTypes": [
            "text"
        ]
    }
]

It also sometimes says “failed to load”. I am looking for guidance on how to debug this as the error appears to be deep within Monday. It appears to be returning error 500 if I check devtools, but this is an error from Monday’s API which is in turn calling mine: