Where are smtpd.key and smtpd.cert? They are not in your output...
I made a clean installation starting from point zero.
Then I follow the tutorial from point 1 to 4
Then there's a crash.
Then I stop at point 4
Point 6: cd /etc/postfix
mv smtpd.cert smtpd.cert_bak
mv smtpd.key smtpd.key_bak
ln -s /usr/local/ispconfig/interface/ssl/ispserver.crt smtpd.cert
ln -s /usr/local/ispconfig/interface/ssl/ispserver.key smtpd.key
The following morning I forget to create again smtpd.cert and ispserver.key smtpd.key.
I will try at midnight again with theses keys before to see if any change.
In addition, I'll give a try on a second server whith the same config to see what happens.