Restore MySQL DB from files
As provided by the OP:

As stated in here, the way to proceed was to initiate mysql daemon in safe mode, just as follows:


Start server bypassing privilege system
$ sudo mysqld_safe --user=root --skip-grant-tables
Just afterwards, you must be able to connect to your mysql instance by console (using mysql command) or by any gui client (e.g. MySQL Workbench).


My guess is that the database system became broken after the latest update (perhaps an unsafe power off, reboot, or so on...). As for now, my MySQL databases will be placed on the NAS, as MySQL won't be updated there. I'll also schedule backup tasks.