Later, I discovered that when I placed the MarsBar toolbox into the spm8/toolbox/ directory and subsequently added a directory containing both the MarsBar toolbox and the spm8 toolbox (which also included MarsBar within the spm8/toolbox/marsbar directory), the MarsBar toolbox was inadvertently added twice, leading to an invalid contrast error. However, by solely adding the path with the spm8 toolbox, ensuring that the MarsBar toolbox was added only once, the error vanished. Amazing!