Hi Sebastian,
I set this up in a lab for you and think i've found the root cause of the issue (Yes the credential type should be set to "other" not ontap...sorry my bad)
I think the root cause is due to the method and permissions in which the computer account is created.
For example, if you were to use the CLI and the default domain "administrator" account to create the computer:
cluster1::> vserver active-directory create -vserver vserver2 -account-name vserver2 -domain testlab.local -ou OU=NetApp
In order to create an Active Directory machine account, you must supply the name and password of a Windows account with sufficient privileges to add computers to the "OU=NetApp" container within the
"TESTLAB.LOCAL" domain.
Enter the user name: Administrator
Enter the password:
C:\>dsquery computer -samid vserver2$
"CN=VSERVER2,OU=NetApp,DC=testlab,DC=local"
You will get the following error when attempting to use the WFA certified command "Delete Active Directory Account"

The reason for the error is as stated "The user has insufficent access rights". That is because the Domain Administrator account is assigned permissions to delete computer object when it is created (not the AD service account which is being used to attempt to delete it as below):

To demonstrate this, if you were to follow the process in the AD GUI (dsa.msc) and pre-create the AD computer account manually and assign the "The following user or group can join this computer to the domain" to the AD service account you intend to use for deleting the AD computer account then it is assigned the correct delete permissions on the computer object EG:


Now if i create a vserver for that computer account (re-using the computer object) when joining it to the domain i am able to use the WFA certified command "Delete Active Directory Account" to sucessfully delete the account. EG


So to summarize i think the problem is related to the Active Directory permissions on your computer object (not the WFA command).
If you like i can send you a WFA command which uses ADSI to create the computer object and assign the correct permissions.
Please let me know if you have any questions
/Matt
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