I don't know where I can find the log-file. I got a message from PLESK with this subject:
Cron <root@server>;; /opt/plesk/php/7.0/bin/php -f '/var/www/vhosts/vvheerenveen.nl/httpdocs/cli/akeeba-backup.php' > /dev/null
And this message:
PHP Parse error: syntax error, unexpected '=' in /var/www/vhosts/vvheerenveen.nl/httpdocs/libraries/fof30/Cli/Traits/CustomOptionsAware.php on line 104
Looks like you are right that the CLI runs from php 7.0. Should I contact my hosting-provider to upgrade to a newer php-version?