I have successfully upgraded. The problem in my opinion was only due to the cache. I did a test on a test environment first. In this environment I copied database and site files, changed the addresses to test it on a different domain. i disabled memcached caches and the upgrade finished successfully. On the production environment I did this:
first a backup of everything
then I put it in maintenance,
deactivated memcached cache from prestashop control panel
then on my provider's panel I cleaned and deactivated the direct nginx cache and cleaned the cloudflare cache
i upgraded and no longer had the disappearing menu issue
Now i’m in 1.7.7.0.
ciao Matteo
P.S. I also had two errors on the database but they are not the ones that generate the problem, they are errors on two tables that do not exist (session tables).