This string is written in :
en-GB.lib_joomla.ini
Two possible cases :
- Joomla lang file is not loaded at that moment.
- The string if build with an extra character or escapement that you don't see, but make the problem happening.
I have not searched more, where this message is raised, or constructed on the request. This should be Joomla native I think.
I saw some components (com_messages) wich copy thoses system strings in copy. It is an heresy, and maybe cook should do the same. But as far as I know it has always worked, so I can't see the problem.
I must dig your case.
Can you tell me wich PTL ?