Safari (iPhone, MacBook) gives NSURLErrorDomain:-1017 I face the issue that Safari returns the error: cannot parse response (NSURLErrorDomain:-1017)
I tested it on different iPhones and one MacBook.
iPhone XS &amp; 11 (iOS 14.3). MacBook Pro BigSur.
You should see the same error when visiting https://gamerino.de
Google Chrome does not have this issue!
What are you using to host the website?
The issue is because of:

It looks like your server is offering an upgrade to HTTP/2 even though
the connection is already done with HTTP/2 - which makes no sense. Not
only that, it is explicitly forbidden. From RFC 7540 section 8.1.2.2:

I had a similar issue with a .NET API which was breaking only with Safari/iOS and the solution was pretty similar to HTTP/2 configuration
Please check here:
https://serverfault.com/questions/937253/https-doesnt-work-with-safari
Nginx: Header unset Upgrade; does not work if you proxy your request, use proxy_hide_header      Upgrade; instead.