Impossible to login (after NIS client setup)

["Rocky Hotas" <rockyhotas%post.com@localhost>]

Hello!!
I tried to configure a NetBSD 6.1.4 machine as a NIS client.
The NIS server was Ubuntu 14.04. Maybe the server itself is badly running NIS for some reason, but this would be off-topic.
However, now I can't login into NetBSD with any username, not even with root.
After boot, the system shows some messages like

Unknown username "root" in message bus configuration file
Could not get password database information for UID of current process: User "???" unknown or no memory to allocate password entry.
[...]
Failed to start message bus: Could not get UID and GID for username "dbus"

and so on. Any login attempt with any username or any password fails.
The server was configured to export only users with UID starting from 1000, so root at least should have not been affected by NIS.
If I disconnect network, the machine repeatedly prints these messages:

ypbind[181]: direct: sendto: Can't assign requested address
ypbind[181]: No contactable servers found in /var/yp/binding/nislocaldomain.ypservers

and doesn't show the login prompt.
With this guide:

https://www.netbsd.org/docs/guide/en/chap-misc.html#chap-misc-forgot-root-password

after running the suggested previous commands successfully, the output of "fsck -u /usr" was

fsck: cannot get disklabel for '/usr': Inappropriate ioctl for device

and "mount /usr" failed saying "unknown special file or file system".
Is there another way to recover the system, at least its files?
And which guide do you suggest for configuring a NIS client with a Linux NIS server?
Thank you anyway,

Rocky