Actually, it's not related to a specific extension. For example, when I disable the RSMediaGallery extension (id=220), the submenu will show correctly. If I leave it active and disable menu id 697, which is linked to an item type related to the Hikashop extension, the submenu will also show correctly. Therefore, I believe the issue is related to a lack of memory. I tried to increase the memory limit in php.ini up to 256M, but nothing changed.