Unable to pick client prefix: no prefixes available

[Yannis Nikolopoulos]

Hello,

I'm getting a lot of "Unable to pick client prefix: no prefixes 
available" errors for subnet6 declarations, where plenty of prefixes are 
still available. Here's an example:

subnet6 2a02:xxxx:49ff:666::/64 {
  option dhcp6.map-option
00:59:x:x:x:x ...
  prefix6 2a02:xxxx:4900:: 2a02:xxxx:493f:ff00:: /56;
}

For the above subnet6, at the time that I'm getting the repeated error 
messages, I can see approximately 8k entries in the leases file (vast 
majority "active").

I'm running isc-dhcpd-4.2.5 on Centos. Am I missing something?

Thanks,
Yannis