This script is provided as is and with-out warranty. Use at your own caution and risk.
BACKUP DATA BEFORE GOING ON!

The current version of Plesk migrator does not migrate address books of roundcube webmail users. This script aims to solve this issue by

• adding missing contacts,
• add missing contactgroups,
• match contacts with groups
• and update the identities table (e.g. with the users' signatures).

(!!! Test if users have been created in users-table. Userwise import manually. @TODO)

• Create a dumb of roundcubemail database from the original server and save it as roundcubemail.sql.
• New Server: Login with ssh and place the migrate_roundcube.php and roundcubemail.sql in a folder.
• Replace the username and password to access the database within the credentials settings in migrate_roundcube.php. (Around line 50-60 within setCredentials()).
• Run script: php migrate_roundcube.php.

Tested for roundcube-version: 2019092900 (03.Nov 2021).

https://support.plesk.com/hc/en-us/articles/213375129-How-to-connect-to-a-MySQL-server-on-a-Plesk-for-Linux-server-using-a-MySQL-admin-password-in-plain-text Example:

# cat /etc/psa/.psa.shadow
$AES-128-CBC$ZmY/EEpy1+TwCNq5kalqSA==$Pd02kf4TTlpXdi/qyeo92w==

add the terminal output inside migrate_roundcube.php (approx. line 50).
!!! Heads up: Make sure to use single quotes.

(...)
    private function setCredentials()
    {
        $this->host = "localhost";
        $this->db_user_name = "admin";
        $this->db_user_pass = '$AES-128-CBC$ZmY/EEpy1+TwCNq5kalqSA==$Pd02kf4TTlpXdi/qyeo92w==';
        $this->dbname = "roundcubemail";
        $this->dbname_source = "roundcubemail_import";
        $this->dump_file = "roundcubemail.sql";

    }
(...)

For Ubuntu 20

# apt install php7.4-cli
# apt-get install php-mysql