Running this with my domain:

sudo certbot certonly --standalone --preferred-challenges http --server  --register-unsafely-without-email -d example.com

Saving debug log to /var/log/letsencrypt/letsencrypt.log
Plugins selected: Authenticator standalone, Installer None
Obtaining a new certificate
Performing the following challenges:
http-01 challenge for example.com
Waiting for verification...
Challenge failed for domain example.com
http-01 challenge for example.com
Cleaning up challenges
Some challenges have failed.

IMPORTANT NOTES:
 - The following errors were reported by the server:

   Domain: exxample.com
   Type:   connection
   Detail: correct.ip.address: Fetching
   
   Timeout during connect (likely firewall problem)

   To fix these errors, please make sure that your domain name was
   entered correctly and the DNS A/AAAA record(s) for that domain
   contain(s) the right IP address. Additionally, please check that
   your computer has a publicly routable IP address and that no
   firewalls are preventing the server from communicating with the
   client. If you're using the webroot plugin, you should also verify
   that you are serving files from the webroot path you provided.

Note: Without using --standalone I am able to download the file wget

I have port forward working correct and in the Ubuntu server I did ufw 80 and ufw 443. But still note, that I am able to wget the file manually.

I have ipv4 records, but no AAAA ipv6 records - but I guess it should work with ipv4.

And I have no problem serving pyton3 -m http.server 80 and curl to the server.

Am I missing something?