PHP cURL OpenSSL SSL_read: SSL_ERROR_SYSCALL, errno 10054

I’m trying to run the PHP cURL request example of an API from a Windows/IIS web server
Here is the code:

<?php
$curl = curl_init();
curl_setopt($curl, CURLOPT_SSLCERT, 'C:\...\cacert.pem');
curl_setopt($curl, CURLOPT_VERBOSE, true);
$streamVerboseHandle = fopen('php://temp', 'w+');
curl_setopt($curl, CURLOPT_STDERR, $streamVerboseHandle);
curl_setopt_array($curl, [
    CURLOPT_URL => $url,
    CURLOPT_RETURNTRANSFER => true,
    CURLOPT_ENCODING => "",
    CURLOPT_MAXREDIRS => 10,
    CURLOPT_TIMEOUT => 30,
    CURLOPT_HTTP_VERSION => CURL_HTTP_VERSION_1_1,
    CURLOPT_CUSTOMREQUEST => "POST",
    CURLOPT_POSTFIELDS => $body,
    CURLOPT_HTTPHEADER => [
        "Authorization: Basic ".$username.":".$password,
        "Message-Reference: SOME_STRING_VALUE",
        "Message-Reference-Date: SOME_STRING_VALUE",
        "Plugin-Name: ",
        "Plugin-Version: ",
        "Shipping-System-Platform-Name: ",
        "Shipping-System-Platform-Version: ",
        "Webstore-Platform-Name: ",
        "Webstore-Platform-Version: ",
        "content-type: application/json"
    ],
]);
$response = curl_exec($curl);
$err = curl_error($curl);
curl_close($curl);
if ($err) {echo "cURL Error #:" . $err."<br>";} else {echo $response;}
rewind($streamVerboseHandle);
$verboseLog = stream_get_contents($streamVerboseHandle);
echo "cUrl verbose information:\n", 
"<pre>", htmlspecialchars($verboseLog), "</pre>\n";
?>

In my php.ini I have the line:

curl.cainfo="C:\...\cacert.pem"

But for some reason I had to add the CURLOPT_SSLCERT option because I was getting:

OpenSSL SSL_read: SSL_ERROR_SYSCALL, errno 10054

I downloaded the cacert.pem file from here as written in this note

But I get this:

cURL Error #:unable to set private key file: 'C:\...\cacert.pem' type PEM
cUrl verbose information:
*   Trying xxx.xxx.xxx.xxx...
* TCP_NODELAY set
* Connected to api.domain.ext (xxx.xxx.xxx.xxx) port 443 (#0)
* ALPN, offering http/1.1
* unable to set private key file: 'C:\...\cacert.pem' type PEM
* Closing connection 0

Am I missing something? How can I solve?

UPDATE
I moved the cacert.pem to the php directory and deleted the CURLOPT_SSLCERT option and now the connection closes later but I get OpenSSL SSL_read: SSL_ERROR_SYSCALL, errno 10054 again:

* Hostname api.domain.ext was found in DNS cache
*   Trying xxx.xxx.xxx.xxx...
* TCP_NODELAY set
* Connected to api.domain.ext (xxx.xxx.xxx.xxx) port 443 (#0)
* ALPN, offering http/1.1
* Cipher selection: ALL:!EXPORT:!EXPORT40:!EXPORT56:!aNULL:!LOW:!RC4:@STRENGTH
* successfully set certificate verify locations:
  CAfile: C:\Program Files (x86)\iis express\PHP\v7.0\cacert.pem
  CApath: none
* SSL connection using TLSv1.2 / ECDHE-RSA-AES128-GCM-SHA256
* ALPN, server did not agree to a protocol
* Server certificate:
*  subject: C=**; ST=********; L=********; O=********; CN=********
*  start date: Apr 12 03:17:12 2024 GMT
*  expire date: Apr 12 03:16:12 2025 GMT
*  subjectAltName: host "api.domain.ext" matched cert's "api.domain.ext"
*  issuer: C=**; O=********; CN=********
*  SSL certificate verify ok.
> POST /path HTTP/1.1
Host: api.domain.ext
Accept: */*
Accept-Encoding: deflate, gzip
Authorization: Basic ********:********
Message-Reference: SOME_STRING_VALUE
Message-Reference-Date: SOME_STRING_VALUE
content-type: application/json
Content-Length: 1602
Expect: 100-continue

* OpenSSL SSL_read: SSL_ERROR_SYSCALL, errno 10054
* stopped the pause stream!
* Closing connection 0

Reading this solution API causes "Curl error: OpenSSL SSL_read: SSL_ERROR_SYSCALL, errno 10054" I also tried from another server without anything in the middle to exclude firewalls problems but same

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