Hi there
Your problem was into the search module published in head-search position it was having the search button set to yes, so despite button was not rendered it was taking space to move mainnav bottom
I changed it now and it is working fine-
P.S.
Regarding your custom-html module "Riferimenti in alto" i noted it is using a "-we" custom class suffix , testing it to see if the reason were that class i haven’t found any class named in that way – in any css file now your module it is showing in the correct way because it is taking the whole position space just after the "languageswitcherload" position.
Best regards