AVOID SOFTACULOUS
In several Hosting providers (specially those under EIG [Endurance International Group]), the Softaculous tool inserts some forced plugins to the Wordpress installations that can cause problems in the websites.
Specifically the plugin -Endurance Page Cache- leaves a problematic trace when being removed, which today we couldn't remove using any of the other solutions mentioned until now in this same StackOverflow question.
--> Finally we proceeded to INSTALL A CLEAN wordpress downloading the most recent zip from wordpress.org, manually creating the new database, extracting the zip in the domains folder and following the installation instructions directly in the website (5 minutes in total). This solved the issue immediately.
Note: As we had a finished development ready we only needed to install a backup from the other installation using our favorite backup plugin. We could have made the migrations by the Zip+SQL procedure, but curiously that procedure broke down some other plugins functionality.
As Microsoft Windows's CrowdStike showed the world this last days, sometimes it's better to avoid centralized platforms, and prefer more trustworthy solutions, as OpenSource (original Wordpress, Linux [we use Manjaro], Apache, MySQL, PHP, etc).