"Not saved locally due to invalid HTML content"

Issue Summary

Like others in https://drupal.org/node/1049594 , I got a "Missing argument 2 for theme_l10n_client_message()" error. I wanted to dig a little bit, so I hacked l10n_client.module to make that second argument optional, defaulting to a WATCHDOG_INFO error level.

It worked: in the translation window, in the Source column I got the message "Not saved locally due to invalid HTML content." The original text was:

<div id="sponsorsHeader">Thank you to our sponsors for their support:</div> <h3><span>Major Sponsors</span></h3>

I entered a translation as:

<div id="sponsorsHeader">Merci de leur appui à nos commanditaires&nbsp;:</div> <h3><span>Commanditaires majeurs</span></h3>

and when I clicked the "Save translation" button (displayed in French), the invalid-HTML error appeared. (If it helps, the original string came from a view: Views 2, Page display, Header, Full HTML format.) What could be causing this?