The error I get is
??? Index exceeds matrix dimensions.
Since PT_mj is a [3*4] matrix and we are trying to access PT_mj(i,j) where i =4, I get this error. I take it you're getting a different error then?
prevents the error, but since I can't get my head around the intended behaviour of the code I doubt if this is really a fix. Especially since with this "fix, " deltalambda_2 is now all zeroes, and that line could be further resolved to
which is a kind of unlikely trivial simplification unless the algorithm is totally misspecified. Without understanding more fully the context, I can't make any progress myself. Maybe you'll have more luck.