My maths is a bit poor, but are the dimensions of the matrices incorrect for multiplication? I seem to remember if you write down [ rows : columns ] x [ rows : columns ] then the two inner numbers have to be the same.

In this case:

[ 71 : 35] x [ 35 : 7 ] x [ 35 : 35 ] x [ 35 : 7 ]
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So, the B x C operation doesn't work. Have you tried replacing these by square matricies all of the same size, or just doing A x B x D?