(resolved) Upgraded to Ruby 2.4.3-1, 'unless' tag was never closed included

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I have a site that runs on Github Pages successfully (and has for several months). Due to a computer issue, I had to re-install Ruby/jekyll/all gems, and installed Ruby 2.4.3-1. I am on Windows, and installed the DevKit at the end of my install. Unfortunately I do not remember which version I was on previously.

Now, when I use bundle exec jekyll serve as I did before, I get the following error message:

jekyll 3.6.2 | Error: Liquid syntax error (~dir/_includes/post.html line 5): 'unless' tag was never closed included

The code has not changed and is currently running happily on Github’s servers. Moreover, when I delete the code section that involves the unless statement, the error still occurs, so I do not believe it has anything to do with the code itself, but it is copied below for completeness:

{% assign post = include.post %}
{% assign link_title = include.link_title %}
<article id="post{{ post.id | replace:'/','-' }}" class="page post{% if link_title %} mb6{% endif %}" role="article">
  <header>
    <h1 class="post-title">
      {% if link_title %}<a href="{{ post.url | relative_url }}" class="flip-title">{% endif %}
        {{ post.title }}
      {% if link_title %}</a>{% endif %}
    </h1>
    <p class="post-date heading">
      {% assign post_format = site.data.strings.date_formats.post | default:"%d %b %Y" %}
      <time datetime="{{ post.date | date_to_xmlschema }}">{{ post.date | date:post_format }}</time>
      {% assign category_start     = site.data.strings.category_start     | default:"using " %}
      {% assign tag_start          = site.data.strings.tag_start          | default:"on " %}
      {% assign category_separator = site.data.strings.category_separator | default:", " %}
      {% assign tag_separator      = site.data.strings.tag_separator      | default:", "  %}
      {% include tag-list.html tags=post.tags meta=site.featured_tags start_with=tag_start separator=tag_separator %}
    {% include cat-list.html cats=post.categories meta=site.featured_categories start_with=category_start separator=category_separator %}
    </p>
    {% include message.html text=post.description hide=page.hide_description %}
  </header>
  {% unless include.excerpt %}
    {{ post.content }}
  {% else %}
    {{ post.excerpt }}
    {% capture post_title %}<a class="heading flip-title" href="{{ post.url | relative_url }}">{{ post.title }}</a>{% endcapture %}
    {% assign text = site.data.strings.continue_reading | default:"Continue reading <!--post_title-->" %}
    <footer>
      <p class="read-more">
        {{ text | replace:"<!--post_title-->", post_title }}
      </p>
    </footer>
  {% endunless %}
</article>

What should I try to fix this?

(Edit)
Setting excerpt_separator: '<!-- more -->' in _config.yml was successful