Dhcpv6 pd example? error "network mask too short"

General questions
4

This works for me:

configure
set interfaces ethernet eth0 address 2001:db8::1/64
set service dhcpv6-server shared-network-name LAN subnet 2001:db8::/64 prefix-delegation start 2001:db8:1:: stop 2001:db8:1:fff0::
set service dhcpv6-server shared-network-name LAN subnet 2001:db8::/64 prefix-delegation start 2001:db8:1:: prefix-length 60
commit
exit

Note that the stop address is inclusive.