Lets Encrypt + mpm-itk
There's a user permission problem with lets encrypt and mpm-itk.
Can you please add/modify following section in /etc/apache2/site-enabled/000-ispconfig.conf
[..]
<Directory /usr/local/ispconfig/interface/acme/.well-known/acme-challenge>
Require all granted
<IfModule mpm_itk_module>
AssignUserId www-data www-data
</IfModule>
</Directory>
[..]