NIS & Shadow problem

[Clemens Heuberger <cheub@weyl.math.tu-graz.ac.at>]

I' m using a NIS and Shadow. Sometimes, I cannot login to the Client, and
the syslog shows 
 
 login[1860]: no shadow password for `cheub' on `ttyp7' from `thue' Sep 17
 login[1860]: invalid password for `cheub' on `ttyp7' from `thue'

If I issue a yppasswd (which is possible since I am logged in under X
Windows) and change my password, I (and all other users) can login again.

Any ideas?

Thanks in advance.

Clemens Heuberger

System Information:
Debian 2.0r2
Linux 2.0.35
  


# /etc/nsswitch.conf
#
# Example configuration of GNU Name Service Switch functionality.
# Information about this file is available in the `libc6-doc' package.

passwd:         compat
group:          compat
shadow:         nis files

hosts:          files dns
networks:       files

protocols:      db files
services:       db files
ethers:         db files
rpc:            db files

netgroup:       nis


-----
/etc/shadow:
----

root:PASSWORD:10331:0:99999:7:::
daemon:*:10036:0:99999:7:::
...
qmailp:*:10036:0:99999:7:::
nobody:*:10036:0:99999:7:::
+::::::::

-----
/etc/passwd:
-----

root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/bin/sh
bin:x:2:2:bin:/bin:/bin/sh
sys:x:3:3:sys:/dev:/bin/sh
...
qmailp:x:76:65534:qmail pw:/var/qmail:/bin/sh
nobody:x:65534:65534:nobody:/tmp:/bin/sh
+::::::