Okay I know this is very old, but since there was no final answer and I just hit something much the same I wanted to share what I found.
Using
radio.setDataRate( RF24_250KBPS );
radio.openWritingPipe(slaveAddress);
and the sketch works. Omitting the "radio.setDataRate" causes the same issue, TX Failed.
This is contrary to the library source code which states that the speeds will be set with the radio.begin (lines 659 - 661 of RF24.CPP.
So regardless of what the docs say, you will need to set the DataRate for the tx to work.