I have 4GB, but I suspect that's not the problem. To test this, I duplicated each of the five original tiffs, making ten. NX2 can open 6 of them (the same ones as before, as well as their copies). If I duplicate each of the original tiffs twice, I can open 9 of them (see screenshot). Would this happen if there were a memory problem?
Interestingly, copies of the tiffs that won't open also won't open. This seems to implicate the tiffs themselves, rather than NX2.