Unfortunately it doesn’t help.
Tried it with a fresh Chrome installation and it end up with the same problem.
I found a solution for Firefox:
"If you’re trying to browse to a website and you’re getting the error NS_ERROR_NET_INADEQUATE_SECURITY in Firefox and you’re not an administrator of the server, you can disable HTTP/2 in Firefox.
Open Firefox and type about:config in the address bar
Click on I Accept The Risk
Search for network.http.spdy.enabled.http2
Change the value to False
Restart your browser "
No not at all. The Site works fine with Firefox but not with Brave and Chrome. Of course I can use Firefox, just triing to find out what the Problem is…