I won (YES)
I reinstalled a Debian 8 - had problems with the NET-IDNA2 package, but manually installed this in /usr/share/php - the rest via apt install.
I then got a roundcube webmail in the old version with dovecot and postfix.
Then I restored the config.php and the whole /etc/mysql and /var/lib/mysql
After adding user accounts I could log in and see the old address books and identities.
Phew....