ESP32 WiFiClient::flush() crashes

Hello,

My following code ( on a ESP32-Oled-Lora module) never comes back from the call :
String response = http.getString();

#include <WiFi.h>
#include <HTTPClient.h>
 
const char* ssid = "******************";
const char* password =  "**************";
 
void setup() { 
  Serial.begin(115200);
  delay(2000);   //Delay needed before calling the WiFi.begin
 
  WiFi.begin(ssid, password);  
  while (WiFi.status() != WL_CONNECTED) { //Check for the connection
    delay(1000);
    Serial.println("Connecting to WiFi..");
  }
   Serial.println("Connected to the WiFi network");
}
 
void loop() {
    if(WiFi.status()== WL_CONNECTED){   //Check WiFi connection status
    HTTPClient http;   
    http.begin("http://mainrouter:85/wf.php");  
    http.addHeader("Content-Type",  "application/x-www-form-urlencoded");               //Specify content-type header
    int httpResponseCode = http.POST("data=abc&targetfile=ds.txt");                     //Send the actual POST request
    if(httpResponseCode>0){
        Serial.print("httpResponseCode>0 : ");
        Serial.println(httpResponseCode);   //Print return code
        String response = http.getString();                                             // <- NEVER COMES BACK !!!!!!!!!!!!!
        Serial.println(httpResponseCode);   //Print return code
        Serial.println(response);           //Print request answer
    }else{
        Serial.print("Error on sending POST: ");
        Serial.println(httpResponseCode);
    }
    http.end();  //Free resources
  }else{
     Serial.println("Error in WiFi connection");   
  }
   delay(5000);  //Send a request every 5 seconds
}

It`s an error, very similar to the one, discussed here :https://forum.arduino.cc/index.php?topic=533282.0
By debugging it down, I found, that WiFiClient.getString() calls the end() function, which calls the flush()-function.
In the flush() function is a while-loop, which never ends ; in my case, a=2 and res=0, so 2-0 will stay forever 2. see code here

// Though flushing means to send all pending data,
// seems that in Arduino it also means to clear RX
void WiFiClient::flush() {
    int res;
    size_t a = available(), toRead = 0;
    if(!a){
        return;//nothing to flush
    }
    uint8_t * buf = (uint8_t *)malloc(WIFI_CLIENT_FLUSH_BUFFER_SIZE);
    if(!buf){
        return;//memory error
    }
    while(a){                                                                   // WHILE NEVER BREAKS   a=2
        Serial.print("Start : WiFiClient::flush()    while(a)  a : "); 
        Serial.println(a);   
        toRead = (a>WIFI_CLIENT_FLUSH_BUFFER_SIZE)?WIFI_CLIENT_FLUSH_BUFFER_SIZE:a;
        res = recv(fd(), buf, toRead, MSG_DONTWAIT);                            // AND res = 0
        Serial.print("res = recv(fd(), buf, toRead, MSG_DONTWAIT) res : "); 
        Serial.println(res);   
        if(res < 0) {
            log_e("%d", errno);
            stop();
            break;
        }
        a -= res;                                                               // SO IT CALCULATES 2-0 !!!!!!!
    }
    free(buf);
}

If I change the line
if(res < 0) {
to
if(res <= 0) {
it seems to work ; but I don’t feel well, because I don’t know, what other sideffects follow….
By the way : When working, I get the message back :

Warning: file_put_contents(…/Ws/Wd/): failed to open stream: Is a directory in /tmp/mnt/USBTomato/Ws/wf.php on line 10

FAIL. Could not connect to file.
So , if someone knows, why my php-script receives an empty stsing, and not as foreseen the String in http.POST(“data=abc&targetfile=ds.txt”), I would be happy to solve this also

Happy Christmas
Daniro