my Cloud Server is reachable via HTTP. In order to make HTTPS work, I created my own certificate and got the response:
Congratulations! Your certificate and chain have been saved at:
/etc/letsencrypt/live/ericsowncloud.ddns.net/fullchain.pem
Your key file has been saved at:
/etc/letsencrypt/live/ericsowncloud.ddns.net/privkey.pem
In /etc/apache2/sites-available/default-ssl.conf I changed the entries as follows:
Visible Content: Index of / NameLast modifiedSizeDescription nextcloud/2018-06-11 10:22 - Apache/2.4.25 (Raspbian) Server at ericsowncloud.ddns.net Port 80
The “Visible Content” (all Html-elements removed) connecting your server via port 443 - is answered via port 80, not via port 443.
Do you have a router?
Does this router send port 443 to port 80?
Your router must send incoming port 443 to your 443 port of your Apache.
This is the really important detail—the server is speaking HTTP instead of HTTPS on port 443. You can probably tell whether the problem is on the server or on the router by trying to connect from inside the router.
(this might cause some difficulties if the server has a different IP address as seen from the inside and outside and if this difference isn’t reflected in DNS records as seen from those perspectives)
Oh, that's bad. But you may have an inconsistent system, so it's good to fix that.
Sometimes there are users with orphan-Apache-processes. An old Apache process doesn't terminate and answers, changes are not visible. A reboot stops that.