I dived deeper into the python code and I found the problem. It is reported on IPFire’s Bugzilla under #12837. Until this is solved, I do not see how the nsupdate provider can be used.
For the moment I use a bash script* to do the job. It was easy to use a dhcpcd exit hook to start the script: it need a link in the right directory. Use
locate dhcpcd | grep hook
to find dhcpcd’s hook directory and place a soft link there pointing to your script. The script needs to react on the reason of a call (positional parameter 1) and gets the IP in positional paramter 2. See the scripts already there for an example, especially what are good reasons to set or delete an DDNS address.
*) I wrote my own, which has some quirks, it’s not (yet?) intended for public use. You may find others that are more robust, tested and supported.