DC in Hyper-V VM after reboot is not working

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http://serverfault.com – I have a Hyper-V Host with a couple of VMs a guest. One of the guest is the DC. There is only on physical server, so I have only one DC as a Vm (I know, it is not a good solution to have only one DC, but we are limited to on physical server). All works fine, until I have to reeboot the Hyper-V host. After a reboot of the host, all VMs are running, but the DHCP-Server in the DC-VM is not giving ip addresses to clients. I have to manually reboot the DC-VM. I have already looked into the event-log, but cannot find some hints. Does anybody know why me DC is not working properly ()