HPUX: lpadmin - can't open member file

problem adding a printer on HPUX (10.20 in my case, but this error occurs on all versions)
lpadmin: can't open member file

one possible cause:
/etc/lp needs to be owned by the 'lp' user id.

# ll -d /etc/lp
drwxr-xr-x   8 root       bin           1024 May  7 15:54 /etc/lp

fix:
chown lp /etc/lp