Hey , thanks ....
So first I just put the #define SERIAL_BUFFER_SIZE 256
in the .ino file before the setup.
And also during the setup I used Serial.println(SERIAL_BUFFER_SIZE); ==> so here it shows 256
But I also tried to change the SERIAL_BUFFER_SIZE directly in the Ringbuffer.h / cpp .... but nothing changed for SERIAL1
That´s why I think there will be another "SERIAL_BUFFER_SIZE" variable for Serial1 ... and the SERIAL_BUFFER_SIZE variable in the Ringbuffer.h/cpp is maybe only for the normal "USB Serial".
So any idea where there might be another SERIAL_BUFFER_SIZE definition for SERIAL1 ... best regards