Confirmed: Uninstalling the USB-00 fileset fix this issue.
The reason why there was no problem prior to hp-ux 11.23 May 2005 release is that the USB-00 fileset release B.11.23.[xy] was installed but not enabled by default. The legacy driver stack was then in use to manage the HID devices. With the May 2005 update, the USB-00 fileset was upgraded to release C.01.00.00 and the provided kernel modules were enabled by default.
Since the USB-00 fileset was there (it has been superseded by USB-02) to enable hardware for future servers, it is useless for my hp workstation zx6000 and can be removed flawlessly. Just ensure that CDE isn't running prior to uninstalling USB-00:
# /sbin/init.d/dtlogin.rc stop
# swremove -x autoreboot=true USB-00
A new kernel is rebuilt and the system rebooted. Everything now runs fine :-)