which is quite disturbing for me and I asked the hosting company – glad that they could help: first, it was an A-record missing which they added and second they changed the htaccess entry to:
RewriteCond %{HTTPS} !on [OR]
RewriteCond %{HTTP_HOST} ^www. [NC]
RewriteRule (.*) https://aschlemmer.net%{REQUEST_URI} [L,R=301]
Thanks to all of you! Solved.