Why this rss feed cannot be imported by any rss reader?

I cannot import this rss feed:

https://mwager.de/feed.xml

On the w3c validator I get the following error:

Server returned [Errno 1] _ssl.c:490: error:14077458:SSL routines:SSL23_GET_SERVER_HELLO:reason(1112)

From: http://validator.w3.org/feed/check.cgi?url=http%3A%2F%2Fmwager.de%2Ffeed.xml

Edit:

I can import the localhost URL though using rss reader app "NetNewsWire":

http://localhost:4000/feed.xml

I just don't get why I can view the feed in a browser but no reader can import it. So this seems like a ssl issue but I have no idea how to tackle this problem...

Edit 2:

If I remove the "force SSL option" on my hosting providers website, it works with the non-ssl URL: http://mwager.de/feed.xml but still not with https://mwager.de/feed.xml