Category Order could not understand your category HTML. Please do the following:

• Disable all plugins except for category order to see if some other plugin is causing the problem. If the problem goes away, re-enable each plugin until you find the incompatible one.
• Try switching to a different theme, like the WordPress default theme.

Once you have tried the above steps, email david@coppit.org with the results of these debugging steps. Also include the following information:

Original HTML:
<li class="cat-item cat-item-5"><a href="https://www.mayuart.com/contents/category/contact/">CONTACT</a>
</li>
<li class="cat-item cat-item-1"><a href="https://www.mayuart.com/contents/category/news/">NEWS</a>
</li>
<li class="cat-item cat-item-4"><a href="https://www.mayuart.com/contents/category/profile/">PROFILE</a>
<ul class='children'>
<li class="cat-item cat-item-19"><a href="https://www.mayuart.com/contents/category/profile/profile-english/">PROFILE-English-</a>
</li>
</ul>
</li>
<li class="cat-item cat-item-17"><a href="https://www.mayuart.com/contents/category/publicity/" title="Publishing information">PUBLICITY</a>
</li>
<li class="cat-item cat-item-3"><a href="https://www.mayuart.com/contents/category/works/">WORKS</a>
<ul class='children'>
<li class="cat-item cat-item-14"><a href="https://www.mayuart.com/contents/category/works/%e7%b5%b5%e6%9c%ac%e3%80%8c%e3%81%bc%e3%81%8f%e3%81%b1%e3%81%90%e3%80%8d/">絵本「ぼくぱぐ」</a>
</li>
</ul>
</li>
<li class="cat-item cat-item-16"><a href="https://www.mayuart.com/contents/category/%e7%b5%b5%e6%9c%ac/">絵本</a>
<ul class='children'>
<li class="cat-item cat-item-20"><a href="https://www.mayuart.com/contents/category/%e7%b5%b5%e6%9c%ac/%e7%b5%b5%e6%9c%ac%e3%82%ad%e3%83%a3%e3%83%a9%e3%82%af%e3%82%bf%e3%83%bc%e3%82%b0%e3%83%83%e3%82%ba/">絵本キャラクターグッズ</a>
</li>
<li class="cat-item cat-item-22"><a href="https://www.mayuart.com/contents/category/%e7%b5%b5%e6%9c%ac/%e8%aa%ad%e8%80%85%e3%81%95%e3%81%be%e3%81%ae%e6%84%9f%e6%83%b3%e3%83%a1%e3%83%83%e3%82%bb%e3%83%bc%e3%82%b8/">読者さまの感想メッセージ</a>
</li>
</ul>
</li>
<li class="cat-item cat-item-18"><a href="https://www.mayuart.com/contents/category/list/" title="Original drawing Pictures list ">絵画作品リスト</a>
</li>
<li class="cat-item cat-item-21"><a href="https://www.mayuart.com/contents/category/%e8%aa%ad%e3%81%bf%e8%81%9e%e3%81%8b%e3%81%9b-%e3%83%af%e3%83%bc%e3%82%af%e3%82%b7%e3%83%a7%e3%83%83%e3%83%97/">読み聞かせ ワークショップ</a>
</li>

Processed HTML:
<li class="cat-item cat-item-5"><a href="https://www.mayuart.com/contents/category/contact/">CONTACT</a>
</li>
<li class="cat-item cat-item-1"><a href="https://www.mayuart.com/contents/category/news/">NEWS</a>
</li>
<li class="cat-item cat-item-4"><a href="https://www.mayuart.com/contents/category/profile/">PROFILE</a>

<li class="cat-item cat-item-19"><a href="https://www.mayuart.com/contents/category/profile/profile-english/">PROFILE-English-</a>
</li>

<li class="cat-item cat-item-17"><a href="https://www.mayuart.com/contents/category/publicity/" title="Publishing information">PUBLICITY</a>
</li>
<li class="cat-item cat-item-3"><a href="https://www.mayuart.com/contents/category/works/">WORKS</a>

<li class="cat-item cat-item-14"><a href="https://www.mayuart.com/contents/category/works/%e7%b5%b5%e6%9c%ac%e3%80%8c%e3%81%bc%e3%81%8f%e3%81%b1%e3%81%90%e3%80%8d/">絵本「ぼくぱぐ」</a>
</li>

<li class="cat-item cat-item-16"><a href="https://www.mayuart.com/contents/category/%e7%b5%b5%e6%9c%ac/">絵本</a>

<li class="cat-item cat-item-20"><a href="https://www.mayuart.com/contents/category/%e7%b5%b5%e6%9c%ac/%e7%b5%b5%e6%9c%ac%e3%82%ad%e3%83%a3%e3%83%a9%e3%82%af%e3%82%bf%e3%83%bc%e3%82%b0%e3%83%83%e3%82%ba/">絵本キャラクターグッズ</a>
</li>
<li class="cat-item cat-item-22"><a href="https://www.mayuart.com/contents/category/%e7%b5%b5%e6%9c%ac/%e8%aa%ad%e8%80%85%e3%81%95%e3%81%be%e3%81%ae%e6%84%9f%e6%83%b3%e3%83%a1%e3%83%83%e3%82%bb%e3%83%bc%e3%82%b8/">読者さまの感想メッセージ</a>
</li>

<li class="cat-item cat-item-18"><a href="https://www.mayuart.com/contents/category/list/" title="Original drawing Pictures list ">絵画作品リスト</a>
</li>
<li class="cat-item cat-item-21"><a href="https://www.mayuart.com/contents/category/%e8%aa%ad%e3%81%bf%e8%81%9e%e3%81%8b%e3%81%9b-%e3%83%af%e3%83%bc%e3%82%af%e3%82%b7%e3%83%a7%e3%83%83%e3%83%97/">読み聞かせ ワークショップ</a>
</li>

Category pattern:
/http\:\/\/www\.mayuart\.com\/contents.*?\/category\/(?:[^\/"]+\/)*([^\/"]+)/

Items:
Array
(
    [0] => <li class="cat-item cat-item-5"><a href="https://www.mayuart.com/contents/category/contact/">CONTACT</a> </li>
    [1] => <li class="cat-item cat-item-1"><a href="https://www.mayuart.com/contents/category/news/">NEWS</a> </li>
    [2] => <li class="cat-item cat-item-4"><a href="https://www.mayuart.com/contents/category/profile/">PROFILE</a></li>
    [3] => <li class="cat-item cat-item-19"><a href="https://www.mayuart.com/contents/category/profile/profile-english/">PROFILE-English-</a> </li>
    [4] => <li class="cat-item cat-item-17"><a href="https://www.mayuart.com/contents/category/publicity/" title="Publishing information">PUBLICITY</a> </li>
    [5] => <li class="cat-item cat-item-3"><a href="https://www.mayuart.com/contents/category/works/">WORKS</a></li>
    [6] => <li class="cat-item cat-item-14"><a href="https://www.mayuart.com/contents/category/works/%e7%b5%b5%e6%9c%ac%e3%80%8c%e3%81%bc%e3%81%8f%e3%81%b1%e3%81%90%e3%80%8d/">絵本「ぼくぱぐ」</a> </li>
    [7] => <li class="cat-item cat-item-16"><a href="https://www.mayuart.com/contents/category/%e7%b5%b5%e6%9c%ac/">絵本</a></li>
    [8] => <li class="cat-item cat-item-20"><a href="https://www.mayuart.com/contents/category/%e7%b5%b5%e6%9c%ac/%e7%b5%b5%e6%9c%ac%e3%82%ad%e3%83%a3%e3%83%a9%e3%82%af%e3%82%bf%e3%83%bc%e3%82%b0%e3%83%83%e3%82%ba/">絵本キャラクターグッズ</a> </li>
    [9] => <li class="cat-item cat-item-22"><a href="https://www.mayuart.com/contents/category/%e7%b5%b5%e6%9c%ac/%e8%aa%ad%e8%80%85%e3%81%95%e3%81%be%e3%81%ae%e6%84%9f%e6%83%b3%e3%83%a1%e3%83%83%e3%82%bb%e3%83%bc%e3%82%b8/">読者さまの感想メッセージ</a> </li>
    [10] => <li class="cat-item cat-item-18"><a href="https://www.mayuart.com/contents/category/list/" title="Original drawing Pictures list ">絵画作品リスト</a> </li>
    [11] => <li class="cat-item cat-item-21"><a href="https://www.mayuart.com/contents/category/%e8%aa%ad%e3%81%bf%e8%81%9e%e3%81%8b%e3%81%9b-%e3%83%af%e3%83%bc%e3%82%af%e3%82%b7%e3%83%a7%e3%83%83%e3%83%97/">読み聞かせ ワークショップ</a> </li>
)