Well, I never really got to the root of the problem and it always bothered me that I had a site without e-mail protection. Today the problem reared its head again and I decided to get to the bottom of it. First stop: forum.jooma.org
Searching for the error message above returned many hits, suggesting that this is a common problem with no immediate solution. After discussing the issue with some other Joomla developers, we quickly discovered that the behaviour only occurred when images were being used as links to the e-mail address. For example:
This was the critical clue and refining our search on the forum confirmed that this type of link was not supported by the email cloaking plugin. Thankfully, user wajdan723 had already posted a hack of the plugin to support image links properly.
Not wanting to hack the Joomla core, Ken Crowder - a colleague and author of Using Joomla: Building Powerful and Efficient Web Sites - quickly took the opportunity to port the hack into a stand-alone plugin called Email Protection Plus that can be installed on your Joomla site as a replacement for the default plugin.
This plugin is now available as a free download here at anything-digital.com .