This is the expected behaviour - the Respond to Webhook node would simply pass through the items it receives, so you can continue with your workflow afterwards if needed.
The reason you are (probably) also seeing this in response to any request sent to your endpoint is the configuration of your initial Webhook trigger node. You want to change the “Respond” option to “Using ‘Respond to Webhook’ Node”:
So I assume this is an error in the tool you are calling your webhook from rather than an error in n8n itself?
My best guess is that your tool is expecting a specific response format that you are not currently sending. But without knowing what your workflow does, or exactly what response your tool expects, it is hard to say. If you could open a new thread and provide all the information necessary to reproduce and understand your problem, that’d be great.