Hi again Andrew,

I didn't get any rank deficiency warning using the design matrix I attached to you in the last message, so I assume it's ok. However, I will follow your advice and remove the task column and assess the task effect in a separate model by using the averaged data of each subject for the two time points. Hope it work. As you said, the two different tasks (MSL and CTL in the attached file) are the same in the two time points.
By the other hand, given that you didn't mention anything regarding the contrasts and the exchange blocks vectors, I assume they are ok.

Thanks you very much for your time and help!

A. D.