Hi Silvio,
your problem comes from a missing localisation. It means that your message is being printed with a <bean:message/> tag by your template and you didn't define an english translation for it.
If you want to localize your application, just define the translations for your messages in the localization panel (by clicking on the "two flags" icon of the menu on the top of the platform).
If you don't want your messages to be localized, be sure that the component which prints it doesn't have the "Localized" flag set to true (it's in the properties panel of the component) and, if this isn't enough, check how your template prints it in the jsp.