@JordyV also, can you tell us what’s in the /etc/yunohost/dyndns folder ?
I investigate this error, and it seems that there is no dnssec key on your server; You can force a key generation by doing :
cd /etc/yunohost/dyndns
dnssec-keygen -a hmac-md5 -b 128 -r /dev/urandom -n USER plumbus.noho.st
chmod 600 /etc/yunohost/dyndns/*.key /etc/yunohost/dyndns/*.private
and then paste us the output of yunohost dyndns update -d plumbus.noho.st --debug.