On Kubuntu, Akonadi is using /usr/sbin/mysqld-akonadi as mysql server. It is symlink to /usr/sbin/mysqld, but in /proc filesystem it is named as mysqld-akonadi...

Akonadi should not expect specific executable name, but should evaluate it from mMysqldPath variable. Moreover, process names in /proc filesystem is trimmed to 15 characters on Linux.

Stop Akonadi

Create symlink /usr/sbin/mysqld-akonadi to /usr/sbin/mysqld, start that process

/usr/sbin/mysqld-akonadi --defaults-file=$HOME/.local/share/akonadi/mysql.conf --datadir=$HOME/.local/share/akonadi/db_data/ --socket=/run/user/$UID/akonadi/mysql.socket --pid-file=/run/user/$UID/akonadi/mysql.pid

Update akonadi server configuration ~/.config/akonadi/akonadiserverrc :

[QMYSQL]
ServerPath=/usr/sbin/mysqld-akonadi

Start akonadi

Akonadi server should re-use existing mysql server...