Thus using the DNS-01 challenge, correct?
If that is the case here is the results https://letsdebug.net/lthlabs.duckdns.org/2391167
I have not checked in 2 or 3 months; but duckdns.org was having some issues,
kind of looks like they still might. 
Edit:
Here is a link to DuckDNS having failures
And still some issues here Hardenize: Comprehensive web site configuration test