In the meanwhile I reconfigured my VPS, so I can not show you what happened in the past hours 
After reconfiguring the VPS I entered the following commands:
apt-get -y update && apt-get -y upgrade
add-apt-repository ppa:certbot/certbot
apt-get install -y apache2 libapache2-mod-php7.0 php7.0-gd php7.0-json php7.0-mysql php7.0-curl php7.0-mbstring php7.0-intl php7.0-mcrypt php-imagick php7.0-xml php7.0-zip mysql-server php-mysql letsencrypt python-letsencrypt-apache redis-server php-redis python-certbot-apache
The system responds:
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:
The following packages have unmet dependencies:
python-certbot-apache : Breaks: python-letsencrypt-apache but 0.4.1-1 is to be installed
python-letsencrypt-apache : Depends: python-letsencrypt but it is not going to be installed
E: Unable to correct problems, you have held broken packages.
What am I doing wrong or what can I do to solve these problems?
Regards,
Guido