[jahn]

I found the solution to my problem.
There is a file called /root/ispconfig/isp/conf/proftpd_ispconfig.conf.master, I had to edit.
Its the template ISPConfig uses to rewrite the /etc/proftpd_ispconfig.conf.
This ist how my /root/ispconfig/isp/conf/proftpd_ispconfig.conf.master looks now:

###################################
#
# ISPConfig proftpd Configuration File
#         Version 1.0
#
###################################

DefaultAddress 127.0.0.1

<!-- BEGIN DYNAMIC BLOCK: vhost -->
<VirtualHost {IP}>
        DefaultRoot             ~
        AllowOverwrite          on
        Umask                   002
        TLSEngine on
        #TLSRequired on
        TLSRSACertificateFile       /etc/proftpd/cert/proftpd.cert.pem
        TLSRSACertificateKeyFile    /etc/proftpd/cert/proftpd.key.pem
        {ANON_FTP}
</VirtualHost>
<!-- END DYNAMIC BLOCK: vhost -->