Fixed. In order, I've done the following:
- I've purged the mysql-server-5.5 and mysql-client-5.5 packages: apt-get purge mysql-server-5.5 mysql-client-5.5
- I've rerun the i-MSCP installer (i-MSCP 1.5.3 version) as follows: perl imscp-autoinstall -dr sql, then when asked, I've choosen the MySQL 5.6 version. For the questions related to the SQL root user and passwod, I've answered with the i-MSCP master SQL user.
Regarding your system
You should really considere to update your system. There are many package updates available. See the output of the apt-get dist-upgrade -s command. If you do so, don't forget to rerun the i-MSCP installer (from the 1.5.3 version) once the packages update is done.