[sync] Activesync with NGINX and PHP7.1: error 500

[Alex]

Hello. I cannot get NGINX and PHP7.1 to work with activesync (everything
else is working with horde). Here is the relevant part from my NGINX config:

location /Microsoft-Server-ActiveSync {
    alias /usr/local/share/horde/rpc.php;
    fastcgi_split_path_info ^(.+\.php)(/.+)$;
    fastcgi_param SERVER_NAME $host;
    fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
    fastcgi_pass unix:/var/run/php/php7.1-fpm.sock;
}

When I open the page "mydomain.tld/Microsoft-Server-ActiveSync" I get the
message:
"Trying to access the ActiveSync endpoint from a browser. Not Supported."

So something seems to be working. However I cannot connect with my phone
(android and iphone). The ActiveSync tester gives the following message:
"Failed to detect ActiveSync [HTTP Status: 500]"

Does anybody have an idea? Please help!