Removing unwanted domain "zentyal-domain.lan" (as hostname!) impossible?

[ATT1]

Hello,

I have configured and installed the domain called "companyname.lan" ; but under "DNS-> Domains" I actually have two domains:
a) Companyname.lan ......  ("redcross symbol" no dynamic domain)  (action: you can delete it)
b) zentyal-domain.lan .....  ("green symbol" :   YES , dynamic dom.) (action: you CANNOT delete it - greyed off)

Why cannot I delete zentyal-domain.lan - do I really need it if I only want "companyname.lan" ?

Self-installed duplicity complains about a domain name and host name mismatch:

root@srv01:/root# duplicity /mnt/data1/shares ftp://ba1@192.168.0.230/shares_backup
NcFTP version is 3.2.5
Synchronizing remote metadata to local cache...
Copying duplicity-full-signatures.20151009T161747Z.sigtar.gpg to local cache.
Copying duplicity-full.20151009T161747Z.manifest.gpg to local cache.
Copying duplicity-inc.20151009T161747Z.to.20151010T131843Z.manifest.gpg to local cache.
Copying duplicity-new-signatures.20151009T161747Z.to.20151010T131843Z.sigtar.gpg to local cache.
Last full backup date: Fri Oct  9 18:17:47 2015
Fatal Error: Backup source host has changed.
Current hostname: srv01.zentyal-domain.lan
Previous hostname: srv01."companyname".lan

How do I change the hostname back to "companyname.lan" and NOT to "zentyal-domain.lan" ?
Preferably an easy way without deleting all users etc?

[trysomething]

On the left side of the web GUI there's a DOMAIN option - you can change it in there.  The biggest problem is that you're going to likely end up with Open Change provisioning errors, network conflicts and other fun stuff and things.
So, here's the really cool part about it though, the .lan domain is only internal, nobody ever really even needs to know the name of that aside from the I.T. gang.  You can still have a zillion virtual mail domains and another zillion web domains via the mail and DNS options.