No Items In RSS Feed

I added a WP BLOG to an existing website, a german fitness website: (fitnesswelt.com)
The configuration is as follows: I installed wordpress under /blog/ and permalinks are pointing to root /.
Feed should be available here: https://www.fitnesswelt.com/blog/feed/ – but there are no items. I was looking in the feed-rss2.php – have_posts() delivers no value (normally it should be true).
But the loop in general is running, as you van see here – you see the loop in the left navigation:
https://www.fitnesswelt.com/run-with-the-sun-%E2%80%93-mallorca-marathon-77-info.html
May be it has to do, that my default page is index.html and not index.php? Index.php will only be found if there is no index.html or if the link points directly to index.php.
Do you have any hint where the problem is?
Thank you,
Torsten