Have you previously added this machine or another machine under the same name or is this some form of upgrade scenario from a previous version of zentyal?
In any case just in case this is a stale reference to the machine you can try completely removing any details from the samba LDAP db with the following command.
sudo ldbdel -H /opt/samba4/private/sam.ldb " CN=<YOUR WINDOWS MACHINE HOST NAME IN CAPS>,CN=Computers,DC=yourdomain,DC=xyz"
ie if your machine was grumpyknackker.wibbly.lan then it would be "CN=GRUMPYKNACKKER,CN=Computers,DC=wibbly,DC=lan"
You can also see if there are any other references to your machine with the following command:
sudo ldbsearch -H /opt/samba4/private/sam.ldb "*" | grep -i <your machine name>
Following this drop your machine out to a workgroup (if it's in some other lan) then reattatch it under a different name