There are options for the strong auth problem but it usually doesn’t affect Nextcloud.
Just to clarify: You are using a local active directory account provider or do you connect to a remote AD?
Maybe there is still something wrong with the account provider settings:
config show nsdc
config show sssd
config show dns
Another method is to try a manual join:
http://docs.nethserver.org/projects/nethserver-devel/en/v7/nethserver-dc.html#manual-join