The problem is .well-known has to be accessible via http.
# curl "http://borbach.com/.well-known/acme-challenge/ysITPUa8hTIqNhE7FjrKF9WjF0QDi4AUgdjhK5ioL3U"
<html>
<head><title>301 Moved Permanently</title></head>
<body>
<center><h1>301 Moved Permanently</h1></center>
<hr><center>nginx</center>
</body>
</html>
As you can see requests for http get redirected to https so SSL verification will always fail. I disabled redirection to https and now it returns the contents of the verification file instead of redirecting to https.
# curl "http://borbach.com/.well-known/acme-challenge/ysITPUa8hTIqNhE7FjrKF9WjF0QDi4AUgdjhK5ioL3U"
ysITPUa8hTIqNhE7FjrKF9WjF0QDi4AUgdjhK5ioL3U.JYseh45lQBxj41K6qwq7KQASWpf6M7NBo1lNHke015E
Here is the recommended way to redirect to https, while making an exception for .well-known to be accessed via http
RewriteEngine On
RewriteCond %{HTTPS} !=on
RewriteCond %{THE_REQUEST} !/.well-known/(.*)$ [NC]
RewriteRule ^ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]
I added the above code to your .htaccess and I reissued Let's Encrypt via Plesk for you and now everything is working as expected.
https://www.sslshopper.com/ssl-checker.html#hostname=borbach.com