mtdshare:
If I add a delay of for example 1 millisecond, the "0" disappears in the Serial Monitor.
But I don't understand why there is a delay needed...
Neither do I. Interesting. At least you have a work-around 
I don't have the time to set up a test at the moment to see if I get the same result.
What happens if you change
bool ack = radio.write(&rxTxData, sizeof(long));
to
bool ack = radio.write(&rxTxData, sizeof(rxTxData));
and
radio.read(&rxTxData, sizeof(long));
to
radio.read(&rxTxData, sizeof(rxTxData));
...R