I inspected the page source and saw that the bottom part is missing. The page ends like this:
<label for="vf-widget-discussions-widget_cache-trigger-post">Post</label>
</div><div style="float:left; display:inline; margin:2px 1px 1px 0;" nowrap="nowrap"><input type="checkbox" id="vf-widget-discussions-widget_cache-trigger-tag" name="vf-widget-discussions-widget_cache-trigger[]" value="tag" /><label for="vf-widget-discussions-widget_cache-trigger-tag">Tag</label>
</div></div></fieldset>
Since ‘vf-widget-discussions’ is related to the Vanilla Forums plugin (I’m using v. 1.0.5), I disabled it, and indeed the missing panels reappeared (well, empty, but at least they’re back).
How can I see the error that causes the page to stop rendering?