Hi Martin,
This message is thrown PHP but it is not a fatal error which blocks the page execution, it should be solved but that kind of messages should not be displayed to users as information about your system (like paths) is exposed to them. This can be managed through debug and debugdisplay settings (you can look for them in the site administration block) http://docs.moodle.org/25/en/Debugging All production sites should disabled debugdisplay.