HP-UX: console (ie telnet) login works, but CDE login fails

This very strange behaviour appeared after setting up a HP-UX 11 machine as NIS client. Only passwd and group are shared by NIS. After doing the appropriate setup with SAM, ypcat passwd and ypcat group worked fine and remote telnet logins to the machine were successful. But remote and locale CDE logins faild with the "usual" CDE communications error.
The solutions is strange, but simple:
Set the Name Service Switch entries "netgroup" and "networks" to NIS as first service and put the files /etc/netgroup and /etc/networks as the second service. Although wether netgroups nor networks are shared by the NIS server, all of our HP-UX 11.00 and HP-UX 11.22 clients worked fine with this setting.