I am requested to install a Linux- Dhcp server in our network for some subnets. After setting up everything, I ran into the following problem. It seems that Webmin does not recognise the service-start during the boot-sequence of ubuntu. If you click start in webmin-panel you start a second process of isc-dhcpd-service.
Reasons for that is a false webmin module-config, which comes with default parameter for an old configuration.

To solve that, you need to modify the module-config to the following parameter.

First open the Module-Config-Page:

Change the highlighted fields to this:

Command to start DHCP server:service isc-dhcp-server start
Command to stop DHCP server:service isc-dhcp-server stop
Command to apply configuration:service isc-dhcp-server restart
Path to DHCP server PID file:/var/run/dhcp-server/dhcpd.pid

Click save. After that everything works for me as expected.