When I want to delete a group this warning appears, when I want to change the hostname on Nethserver a warning also appears?
I am afraid you have to uninstall the account provider, change the hostname and install the account provider again.
From the docs:
Please choose wisely your account provider because the choice could not be reversible. Also the system will forbid any change to the FQDN after the account provider has been configured.
http://docs.nethserver.org/en/v7/accounts.html#account-providers
If I manually set it through the terminal, it will change and be deleted, but why can’t the Nethserver admin be replaced or deleted on the web admin?
This would break your system.
Its a internal system user and the one of the default domain admins, it can not be removed.
AFAIK Same applies for the the administrator
on a system with AD-account provider…
Yesterday I deleted the default user and group admin but why can’t it now?
Is your account provider running properly?
And I can only recommend not to remove the user admin!