NRF52-DK SPI example not working.

I just try to start with the spi master example (nrf5sdk/examples/peripheral/spi) on mus PCA10040 board.
I'm able to compile&flash it. It seems to be running (LED1 is toggling and I see the prints on the debug uart (
<info> app: SPI example started.
<info> app: Transfer completed.
<info> app: Transfer completed.

I just connected my oscilloscope to pin 0.26 to see the sck signal - but I can't see anything happening on this line.

I didn't change anything in the code.
Any Idea what I do wrong?

Thanks in advance.