Login error "wrong password"

installed nextcloud to my Synology NAS, login page for users visable so I think everything should be ok for the first steps of the installation. (I followed a German instruction which was really detailed and I could manage to do all steps).
I have the NAS users and passwords available and can login to my NAS via web page at the special NAS admin port. I can NOT login to the nextcloud web page on port 80, the error is "wrong password, should it be reset?"
So first of all, it recognizes the username, which must come from NAS user list. The reset of the password can not be done, as it complains that the username is not correct. This sounds reasonable as there is no email address for the NAS user.
Version is 9.0.53 fresh install, no migration.
phpMyAdmin is there an I can login with MariaDB root.
What is needed to be done here? Is it needed to create nextcloud users as well, so a double list of users?
There is no data yet, so a fresh install/re-install is an option, but howto?
rgds Jörg

I personally am not familiar with how Synology NAS manages their user accounts, but if it uses any of the supported methods you can find a detailed description here, on how to do this.
Basically, you can have your own nextcloud user/password database entry or use LDAP, SMB, IMAP or FTP