I stumbled across this problem when a daily cronjob on one of our Debian servers complained about this. A lot. If you run across this on one of your systems, there is a quick fix.
In our case, it turned out that
/usr/bin/mandb could not read
because the mode was 0600 rather than 0644. A quick change (
chmod 644 /etc/passwd)
and things were back to normal.