In site-packages\sphinx_rtd_theme > layout.html
After
{% if sphinx_version >= "1.8.0" %}
<script type="text/javascript" id="documentation_options" data-url_root="{{ pathto('', 1) }}" src="{{ pathto('_static/documentation_options.js', 1) }}"></script>
Add
<script type="text/javascript" id="language_data" data-url_root="{{ pathto('', 1) }}" src="{{ pathto('_static/language_data.js', 1) }}"></script>
so that the Stemmer javascript object can be found.