SPI problem with Bitcraze PMW3901 sensors and Arduino NANO

Wrong, for a 4MHz bus frequency the maximum wire length is about 50cm, so your wires are too long.

I found the schematics of the Bitcraze board and unfortunately the board designer doesn't know how to build a correct SPI level converter. The MISO line is constantly pulled low so you cannot connect any other device to the same SPI bus if this board is connected.

IIt would have been quite easy to change that: Just don't connect pin 12 of U1 not to pin 1 but by a voltage divider to pin 10.