Pb with Mega menu - Failed to load resource: the server responded with a status of 403 (Forbidden)

Hi the T3 community
I'm new in this forum and I test the Purity III template

I have an issue with Mega Menu configuration
Sub menu don't want to load

I can see this error in Chrome

Failed to load resource: the server responded with a status of 403 (Forbidden) (http://www.modem-essonne.org/_SD_Circo_modem91_/_SD_circo2_j3/?t3action=megamenu&t3task=display&styleid=10&template=purity_iii&t3menu=mainmenu&t3acl=1&t3lang=*&rand=0.2043533397372812)

an idea ?
thanks

Bertrand