This is the full error I get when I start DDclient (*** = deleted for privacy):

WARNING: file /var/cache/ddclient/ddclient.cache, line 8: Invalid Value for keyword 'ip' = ''
WARNING: skipping update of *** from <nothing> to ***.
WARNING: last updated <never> but last attempt on Tue Jun 20 14:09:37 2017 failed.
WARNING: Wait at least 5 minutes between update attempts.

And this is my ddclient.conf:

# Configuration file for ddclient generated by debconf
#
# /etc/ddclient.conf
daemon=60
syslog=yes
mail=***
mail-failure=***
pid=/var/run/ddclient.pid
ssl=yes

use=web, web=checkip.dynu.com/, web-skip='IP Address'
server=api.dynu.com
protocol=dyndns2
server=dynu.com
login=***
password=***
***.DYNU.NET

I've tried different IP sources as well, such as 'use=web, web=ipinfo.io/ip'

I see that there used to be a bug in ddclient, but I believe that was quite a while ago.

Please try the following config file:

# ddclient configuration for Dynu
#
# /etc/ddclient.conf
daemon=60
syslog=yes
mail=root
mail-failure=root
pid=/var/run/ddclient.pid

use=web, web=checkip.dynu.com/, web-skip='IP Address'
server=api.dynu.com
protocol=dyndns2
login=myusername
password=YOURPASSWORD
MYDOMAIN.DYNU.COM