This time around, my ISP made a routine switch in my home IPv4 address, which my DynDNS software was able to follow, without any intervention on my part. This Earlier Posting explained, that a switch can also take place, which the software fails to follow automatically.
And this time, I did need to update my IPv6 (Teredo) address manually. It is to be assumed that IPv6 address changes of my server need to be updated manually, as explained in This Earlier Posting.
Hence, since last night, my IPv6 was unavailable until 13h15 this afternoon.