It seems that what is needed is not so much an update, as an initial configuration.
When I run the command on the server where I configured a noho.st domain, there’s only very short output:
$ sudo yunohost dyndns update -d frankfurt.noho.st --force
Info: Updated needed, going on...
Success! Updated your IP on DynDNS
$
Yours complains about being refused. Did you enter a recovery password when setting the ynh.fr domain the first time? If not, that could be the problem; if yes, maybe retype it in the configuration:
Edit to add:
There is a somewhat similar issue going on, where metyun suggests running a subscribe command :
$ sudo yunohost domain dyndns subscribe yourdomain.ynh.fr -p yourpassword
Again, the password is the recovery password that you gave in during registration of the domain.