Radiohead pain – manager.available() always false

Hello,
I use successfully a few ‘selfsoldered’ lora modules (Arduino pro mini with hope-lora) with the Radiohead RHReliableDatagram
On my Heltec Oled-ESP-Lora modules, the code doesn`t work. The manager.available() in the main loop returns always false ; by commenting the line out, everything works.

I join the code (shortened to the minimum). I touch my limits, help is highly appreciated. What’s wrong with my manager.available() ?
Thanks in advance !

#include <SPI.h>
#include <RHReliableDatagram.h>
#include <RH_RF95.h>

// Pin definetion of WIFI LoRa 32
// HelTec AutoMation 2017 support@heltec.cn 
#define SDA         4    // GPIO4  -- SX127x's SDA
#define SCK         5    // GPIO5  -- SX127x's SCK
#define SCL         15   // GPIO15 -- SX127X's SCL
#define MISO        19   // GPIO19 -- SX127x's MISO
#define MOSI        27   // GPIO27 -- SX127x's MOSI
#define SS          18   // GPIO18 -- SX127x's CS
#define RST         14   // GPIO14 -- SX127x's RESET
#define DI00        26   // GPIO26 -- SX127x's IRQ(Interrupt Request)

#define CLIENT_ADDRESS 1
#define SERVER_ADDRESS 2

RH_RF95 driver(SS,DI00);                   // Singleton instance of the radio driver
RHReliableDatagram manager(driver, SERVER_ADDRESS);   // Class to manage message delivery and receipt, using the driver declared above

unsigned int counter = 0;

void setup(){
  Serial.begin(115200);
  while (!Serial) ; // Wait for serial port to be available
  //**************** Initialize SPI **********************************//
  SPI.begin(SCK,MISO,MOSI,SS);
  //************ reset lora module **********************************//
  pinMode(RST, OUTPUT);
  digitalWrite(RST, HIGH);  delay(10);
  digitalWrite(RST, LOW);   delay(10);
  digitalWrite(RST, HIGH);  delay(10);
  //****************Initialize lora **********************************//
  while (!manager.init()) { 
    Serial.println(".");
    delay(500);
  }
  Serial.println("LoRa Initializing OK!");
 } //**************** End of Setup **********************************//


void loop(){
    Originalloop();                 // Result: just sends "no manager available" to serial
    WithoutManagerAvailableloop();  // Result: works fine, sends successfully
                                    // or gives "Send failed", if there is no receiver
  }
  
void Originalloop(){
  if (manager.available())  {
      //********************* send packet**************************************
      String testString = "Hello "+String(counter,DEC);
      uint8_t* testStringpointer = (uint8_t*)testString.c_str();
      int testStringlength = testString.length();
      if (!manager.sendtoWait(testStringpointer, testStringlength, CLIENT_ADDRESS)) {
         Serial.println("sendtoWait failed");
      }
      counter++;
 }
  else {
        Serial.println("no manager available");
  }
   delay(1000);                       // wait for a second
}

void WithoutManagerAvailableloop(){  
  //********************* send packet**************************************
  String testString = "Hello "+String(counter,DEC);
  uint8_t* testStringpointer = (uint8_t*)testString.c_str();
  int testStringlength = testString.length();
  if (!manager.sendtoWait(testStringpointer, testStringlength, CLIENT_ADDRESS)) {
     Serial.println("Send failed");
  }
  counter++;
  delay(1000);                       // wait for a second
}