Maybe the long wire from Minimig to ARM board is the cause of this problem.
There are quite high frequence working and some wire may have cross-talk to others or are acting as an "antenna".
I would try to shorten those wires or try to twist them together (like twisted-pair in network cable).
Best solution would be to hook the board directly at the PIC Socket.
Good point, I was wondering if the wire length might be a problem.
I think I'm going to try and redo this with a pin/crimp socket and individually soldered wires.
I've had issues with the push to crimp ribbon sockets in the past and I'll also be able to shorten the wire length this way.
Still weird though, I've used SPI over a much longer cable at high frequency before and never encountered something like this.
Then again, I don't have any deep understanding of this stuff at all