Additional Fields add-on kills Appointments+

I'm trying to add Additional Fields using the addon included with Appointments+. After enabling the addon the General Settings page does not completely render, it fails while building the Additional Fields section.

Here's where it fails:

<script id="app-additional_fields-template" type="text/template">

<div class="app-field">

<b>label</b> <small>(type)</small>

Required: <b><?php echo esc_js(__("Yes", "appointments")); ?></b>
<input type="hidden" name="app-additional_fields[]" value="

(it dies right after the last double quote, there's nothing else rendered to the page)

I've tried uninstalling the plugin (several times). Seems like there's still a setting or something broken in a table somewhere but I can't seem to find it.