Hi @heninng,
This error appears to only happen where content has been imported from a previous version of the Joomla CMS – such as from a Joomla 1.5 to a Joomla 2.5 installation.
The easiest way to get around this error is to disable the ‘Content – Email Cloaking’ plug-in. This plug-in automatically hides email addressed from spambots by encrypting your email address and encrypting when the page is loaded by a normal visit.
The other option to fix this issue appears to be opening each of the affected content pages and clicking ‘save’ again. You may need to do this whilst the email cloaking plugin is enabled to refresh the email address in the content.