start page is not beeing cached

First, thank you for the grat plugin!
All my pages are cached properly, except the start page of the site:

https://www2.karate-usc.de/

In the cache folder the index.html and index.html.gz files are created, but not served by apache.
May be a problem with the .htaccess file? I inserted the lines from https://gist.github.com/sergejmueller/2027249#file-htaccess at the top of the file.

https://www.ads-software.com/plugins/cachify/