Thank you very much for your help. Because you could see correctly what I couldn't, I realized that there was something with the configuration I was using. I was using the display->use filter option set to "no". When set it to "yes", the problem solved , on the condition that I don't use "moodle navigation frame" , "embedded web page" or "original navigation aids" settings in the display->navigation option. Also, with these options and when setting the option display->use media filter to "moodle standard media filters" the following error appears:
Strict Standards: Declaration of hotpot_mediafilter_moodle::mediaplugin_filter() should be compatible with that of hotpot_mediafilter::mediaplugin_filter() in /usr/share/moodle/mod/hotpot/mediafilter/moodle/class.php on line 38.
This error is also triggered when the option display->output format is set to Jmix(V6+) from xml.
Hope this can help, and thanks again