Thank's for all replies to my question about yppasswd not doing a make.
Since a lot of people have posted me asking for answers, I'll post it here.
The problem lies in a bug with yppasswdd not reading it's first parameter
(-m). Therefor the solution was to include a 'dummy'-parameter before this.
I've used the '-nosingle', which really is default.
My rc.local entry:
/usr/etc/rpc.yppasswdd /etc/passwd -nosingle -m passwd
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