I tried to enable the login for the user "root" in the FTP options.
(Services -> FTP -> Settings -> Advanced settings -> "Permit root login")
This adds the user root in the vhosts sections in /etc/proftpd/proftpd.conf
Unfortunately proftpd still blocks the root user from login to the FTP server (using FileZilla)
The /var/log/proftpd/proftpd.log log file:
The file /etc/ftpusers still contains the root user:
-> comment out the line containing "root" and the login for root is working.
=> This should be done automatically if the "Permit root login" switch is enabled