I now downgraded the esp32 library to the version you mentioned and that did the trick. Thank you very much.

I also want to link this post here: https://forum.arduino.cc/t/downgrade-from-v3-0-x-to-v2-0-x/1272211/2
it explains how to downgrade the version and how to turn off the automatic upgrade ...

Here is the new code:

#define F_XTAL_MHZ 26

void setup() {
pinMode(LED_BUILTIN, OUTPUT);
Serial.begin(115200);
delay(5000);
}
void loop() {
digitalWrite(LED_BUILTIN, HIGH);
Serial.println("on");
delay(1000);
digitalWrite(LED_BUILTIN, LOW);
Serial.println("off");
delay(1000);
}


and ...

Thank you for your help.
This is what I got during the upload with debugging

Sketch uses 306564 bytes (23%) of program storage space. Maximum is 1310720 bytes.
Global variables use 20048 bytes (6%) of dynamic memory, leaving 274864 bytes for local variables. Maximum is 294912 bytes.
esptool.py v4 ...

Hello there,

I am using an ESP32 TTGO Lora Oled V1 and uploaded this simple sketch

void setup() {
pinMode(LED_BUILTIN, OUTPUT);
Serial.begin(115200);
delay(5000);
}
void loop() {
digitalWrite(LED_BUILTIN, HIGH);
Serial.println("on");
delay(1000);
digitalWrite(LED_BUILTIN, LOW);
Serial ...