When specifying a range prefix withing a subnet in the dhcpv6-server config section, if the range is of the invalid shape:

2001:aaa:bbb::ccc:/xxx

the VyOS configurator will accept the value but dhcpdv6 won't start and will error out on the range6 line.

This can be fixed by specifying the address as follows (i.e. by adding a trailing 0):

2001:aaa:bbb::ccc:0/xxx

VyOS should detect the invalid IPv6 address and reject it before committing.