I'm accessing a user definable API through a service webhook in Azure DevOps. I can tell from my information event logging that the script runs completely fine. At the end of the script I do this.

        return new ApiTriggerOutput
{
ResponseCode = (int)StatusCode.Ok,
};

But in Azure DevOps I get an internal server error with code 500.

If my request is in an incorrect format, I have some checks in my script that then return something like this:

            return new ApiTriggerOutput
{
ResponseBody = $"Azure DevOps project with id {workItem.resourceContainers.project.id} is not supported",
ResponseCode = (int)StatusCode.BadRequest,
};

These response codes come in correctly into Azure DevOps.

What's happening?

EDIT: Full response:

Status Code: 500
Reason Phrase: Internal Server Error
HTTP Version: 1.1
Headers:
{
Connection: keep-alive
Request-Context: appId=cid-v1:e6146ee8-8f7e-4826-92a9-9e3220cf1975
TunnelConnection: 04f72ccd-573f-4ea4-abf7-aeff6b604a18
TunnelForwarded: True
Strict-Transport-Security: max-age=15552000; includeSubDomains
SL-Filter: ALLOWED (single request, by path), time=2024-2-22 9:10:45
Content-Security-Policy: frame-ancestors 'self'
X-Content-Type-Options: nosniff
X-Frame-Options: deny
Date: Thu, 22 Feb 2024 08:10:45 GMT
Set-Cookie: TunnelConnection=04f72ccd-573f-4ea4-abf7-aeff6b604a18; path=/; secure; httponly
Content-Length: 235
Content-Type: application/json; charset=utf-8
}