E803 - Rendered template output invalid

Cause

A template rendered successfully, but its output is not a valid JSON5 array of elements — malformed JSON5 text, a non-array root, or an array entry that fails element validation.

Example

[ {% for s in steps %} { markdown: "{{ s }}" position: [5, 10] }, {% endfor %} ]

(Missing comma between fields — valid Jinja, broken JSON5.)

How to fix

The error carries an excerpt of the rendered text around the failure; run scrolly expand on the instantiating slide to see the full rendered output, then fix the template so it renders valid JSON5 elements.