So I came across this because I was getting the Internal Error but looking at Post 5 I realized my problem, which is probably most of the DuckDNS people here’s problem as well.
With DuckDNS you need to set Nginx SSL to Use a DNS Challenge & from the DNS Provider list choose DuckDNS
You then need to login to DuckDNS.org & copy your token to the dns_duckdns_token=your-duckdns-token box so it looks something like dns_duckdns_token=123abc-zyxwv9876-1234-abcd-1a2b3c4d5e
Since I was renewing mine I forgot that & I’m guessing those here did as well. Any future lookers as well, since there wasn’t a solution yet hope it helps