What version of Nextcloud did you install?
AFAIK, the default “admin” account for Nextcloud is a local, nextcloud account. When you have installed Nextcloud the password is set as Nethesis,1234 and you have to change this password asap for security reasons.
NethServer also has an “admin” account when you use Samba4 AD account provider. Can it be that you try to log in with that account?