As discussed with Ohad:
When creating a host I get often:

1 error prohibited this host from being saved
 Failed to set the DHCP record: MAC 00:0f:30:6f:da:bc is used by this reservation: 00:0f:30:6f:da:bc - 192.168.222.112

That happens probably because I have turned the machine on before (needed by VMWare to generate the MAC address). Using the same IP as the lease doesn't help with the problem.