I did find out more.
Removing the "TLS file" containing the "1" created with the command below made the pure-ftpd work for me.
echo 1 > /etc/pure-ftpd/conf/TLS
So pure-ftpd seems to work without the TLS, the deamon starts then and a ps-ef|grep ftp shows him, while it does not with TLS on my server.
I think further that there was nothing wrong with the squirrelmail, I tried to login with the administrator account and this works before installing ISPConfig3 but not after installing ISPConfig3. ISPConfig3 allows only email accounts that are created in ISPConfig3, I got squirrelmail working with an email account created in ISPConfig3.
I only wonder why the pure-ftpd does not work with TLS, would still like to know why, but for the purpose I use the server for (not connected on the web but in a safe environment) it works.