Collapsiblock does not collapse when the block has no div class content like in Zen theme.

Can confirm this. Won't work with adaptive theme either.

I am struggling with this as well. Thanks.

When using themes like Zen 7.x-5.x you will either need to modify block.tpl.php (or its derivatives) as shown above or change the collapsiblock selectors at admin/appearance/settings. If certain base themes are going to make it hard to figure out where the block content is located, that's not this module's fault. I suggest mentionning this briefly in README.txt.

forgot to update version

Actually, on Zen theme 7.x-5.5 you need to change this

Change:

<div id="<?php print $block_html_id; ?>" class="<?php print $classes; ?>"<?php print $attributes; ?>>

  <?php print render($title_prefix); ?>
  <?php if ($title): ?>
    <h2<?php print $title_attributes; ?>><?php print $title; ?></h2>
  <?php endif; ?>
  <?php print render($title_suffix); ?>

  <?php print $content; ?>

</div>

Into:

<div id="<?php print $block_html_id; ?>" class="<?php print $classes; ?>"<?php print $attributes; ?>>

  <?php print render($title_prefix); ?>
  <?php if ($title): ?>
    <h2<?php print $title_attributes; ?>><?php print $title; ?></h2>
  <?php endif; ?>
  <?php print render($title_suffix); ?>
<div class="content">
  <?php print $content; ?>

</div>

@VBN: I think you left out the closing div tag. It should be:

<div id="<?php print $block_html_id; ?>" class="<?php print $classes; ?>"<?php print $attributes; ?>>
  <?php print render($title_prefix); ?>
  <?php if ($title): ?>
    <h2<?php print $title_attributes; ?>><?php print $title; ?></h2>
  <?php endif; ?>
  <?php print render($title_suffix); ?>
  <div class="content">
  <?php print $content; ?>
  </div>

</div>

Requires re-architecture, should be tackled in a new version.

See parent issue.