Why would a Static Block that contains an image hang/timeout on save?

Question

I'm troubleshooting an issue with Static Blocks.

When I create a new Static Block with an image, it works fine, but if I then go back into the Static Block to edit it, even without making any changes, it will hang/timeout on save.

Static Blocks without images are working fine.

The error that I'm receiving after about a minute is in a Nginx "502 Bad Gateway", which is closely related to "504 Gateway Timeout".

The code for my Static Block is below.

---

<ul class="slides">
    <li>
        <a href="/some-page">
            <img src="{{media url="wysiwyg/carousel-image.jpg"}}" />
        </a>
    </li>
</ul>

---

Do you have any idea why a Static Block with an image hang/timeout on saving?