@clannad - Thank you for confirming this. By way of final update. I logged back into the Flint2 tonight and found the DDNS daemon had just given up trying anymore. The logs were filled with repeating errors as shown below. This time i restarted the client again and it worked on the first try.
I'm wondering if maybe these few devices that were still refusing to update were running into some kind of cached error-state on your server side from the outage that had to expire before working again. Seem the fix is to just wait long enough and then restart the server's DDNS client (again) and then restart the VPN client one more time.
In the meantime, I think for my clients I'm just going to implement a cron job that restarts the gl_ddns client once per hour on the server to protect against it going into a permanent failed state like this in the future.
~# cat /var/log/ddns/glddns.log
#> lua /usr/lib/gl_ddns/get_ddns_registered_ip.lua >/var/run/ddns/glddns.dat 2>/var/run/ddns/glddns.err
#> lua /usr/lib/gl_ddns/get_ddns_registered_ip.lua >/var/run/ddns/glddns.dat 2>/var/run/ddns/glddns.err
NO valid IP found
NO valid IP found
NO valid IP found
Get registered/public IP for 'qh5733a.glddns.com' failed - retry 143/0 in 60 seconds
Get registered/public IP for 'qh5733a.glddns.com' failed - retry 143/0 in 60 seconds
Get registered/public IP for 'qh5733a.glddns.com' failed - retry 143/0 in 60 seconds
#> lua /usr/lib/gl_ddns/get_ddns_registered_ip.lua >/var/run/ddns/glddns.dat 2>/var/run/ddns/glddns.err
#> lua /usr/lib/gl_ddns/get_ddns_registered_ip.lua >/var/run/ddns/glddns.dat 2>/var/run/ddns/glddns.err
#> lua /usr/lib/gl_ddns/get_ddns_registered_ip.lua >/var/run/ddns/glddns.dat 2>/var/run/ddns/glddns.err
NO valid IP found
NO valid IP found
NO valid IP found
Get registered/public IP for 'qh5733a.glddns.com' failed - retry 144/0 in 60 seconds
Get registered/public IP for 'qh5733a.glddns.com' failed - retry 144/0 in 60 seconds
Get registered/public IP for 'qh5733a.glddns.com' failed - retry 144/0 in 60 seconds
#> lua /usr/lib/gl_ddns/get_ddns_registered_ip.lua >/var/run/ddns/glddns.dat 2>/var/run/ddns/glddns.err
#> lua /usr/lib/gl_ddns/get_ddns_registered_ip.lua >/var/run/ddns/glddns.dat 2>/var/run/ddns/glddns.err
#> lua /usr/lib/gl_ddns/get_ddns_registered_ip.lua >/var/run/ddns/glddns.dat 2>/var/run/ddns/glddns.err
NO valid IP found
NO valid IP found
NO valid IP found
EDIT - also contents of .err:
cat /var/run/ddns/glddns.err
% Total % Received % Xferd Average Speed Time Time Time Current
Dload Upload Total Spent Left Speed
100 36 100 36 0 0 54 0 --:--:-- --:--:-- --:--:-- 66