You need to place both configs in the same VirtualHost block. You can't split it up into multiple configuration files.
Alias /phpmyadmin /usr/share/phpmyadmin
# rest of config ...
... "GET / HTTP/1.1\n" 400 3279 "-" "-"
The line ending in a HTTP request should be \r\n and not \n. Try the -crlf option of s_client to translate an Enter into \r\n instead of only \n.
Also the server might have some limit in which time the request should be fully sent and it might not be fast enough to just type everything. ...
You did actually send a malformed request. HTTP/1.1 requires the presence of the Host: header, and further with SNI, the content of the Host: header must match the SNI hostname.
Try again and this time provide the correct Host: header.
Unfortunatly you can't redirect the Client, without having a valid certificate. As the TLS Handshake happens before the redirect.
Since it seems as you are using Letsencrypt the best solution would be the get a certificate with both Names. If you are using certbot you could just add the -d Domain flag multiple times.
certbot -d yachtenwelt.de -d www....
The main advantage of placing your web site in a virtual host is that the default (virtual) host, which Apache's syntax doesn't place inside any <VirtualHost> block, will catch requests for any site other than yours. This includes things like localhost (if you didn't make a virtual host for it), the system's IP address, no host specified, etc. Nearly ...