Want to switch from Duckdns to own Domain - STUCK

I’m now using duckdns and nginx proxy manager (reverse proxy).
But because google often reports that my HA instance is not available, I want to switch to my own domain.
First, I registered a domain with Strato “mydomain.nl”
and created a sub domain “ha.mydomain.nl”
next I forwarded this domain to my public IP address.
After that I opened my Home Assistant, opened Add-ons, selected NPM > open web UI.
Added a new proxy host “ha.mydomain.nl” and forwarded to port 8123 (the device where my HASS is installed. Checked ‘websockets support’ and saved.
When I click on “ha.mydomain.nl” to test I go to my public IP address and the screes says ‘Congratulations! You’ve successfully started the Nginx Proxy Manager’
Next I try to request an SSL certificate, but this results in an internal error.

I don’t know what I’m doing wrong,… hope someone here is able to help me set this up.