How to go about tracking down this error?

IMHO the 'end' event would be a good addition. Right now it's somewhat painful and error prone to determine when a complete HTTP request has been received.

@allObjects: I would have to disagree with your assessment about what uC's are capable of. I totally agree that an architecture where pretty raw data from many nodes is aggregated makes a ton of sense. But while this is what I like to build myself as well, I also like to make the uC nodes that have actuators stand-alone 'cause there's nothing more frustrating than having your entire house/greenhouse/... being inoperable because the central server crashed or otherwise failed. Or not daring to upgrade the central server 'cause so much now depends on it.

What makes stand-alone http servers practical is the fact that all the UI can be shoved into browser-side JavaScript. I do that with esp-link and the constraints on the UI are pretty minimal at this point. The uC end up exchanging simple json or binary requests with a central uC and the browser alike. All that's needed is enough flash to hold the minified and compressed JS for the UI and be able to ship it to the browser. If you skip large libraries, such as jquery and bootstrap that's actually pretty easy to do.

The upshot of all this is that I'm gravitating towards uC nodes that work with a central node and that have a simple stand-alone UI.