Thu Jan 1 00:00:50 1970: INADYN: Started 'INADYN Advanced version 1.96-ADV' - dynamic DNS updater.
Thu Jan 1 00:00:50 1970: INADYN: IP read from cache file is 'xx.xx.xxx.xx'. No update required.
the problem appears to be a race condition between the new IP address being assigned and the DDNS check (the "thu jan 1" date suggesting the time of day hasn't updated either). a fix is suggested by:
Go to: Administation-> Management
In Additional Cron Jobs write:
*/10 * * * * root /usr/sbin/inadyn /tmp/ddns/inadyn.conf > /dev/null 2>&1
which runs the inadyn command every 10 minutes using cron. i've just applied/saved this setting. the DDNS updated immediately after applying this. since the problem was intermittent, i can't know if it's solved. if the problem reoccurs, i'll update this post