The Loopback test only involves MISO and MOSI lines that leaves CS and CLK 
as potential source of errors.
I have made some measurements with an oscilloscope and it shows two notable 
differences between 4.4 and 3.8:

- For 3.8 CS toggles between high and low. For 4.4 CS is always low after 
an short initial high period. Both scenarios should be correct for the SPI 
data transfer as I see it.
- For 3.8 you can see how data is transferred in the clock cycles but for 
4.4 the MISO line always remains low.

What can cause this, how can I approach this issue further?

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