Dear Martin,
I can't thank you enough for the prompt and professional reply and support. It's really helpful. besides the shading problem, which is the main one, because of your reply I noticed also that the inverter's MPPT range is relatively narrow, which is something I may need to take care of in our next projects. One more thing: It's impressive how the PVSOL evaluates the system losses, particularly considering the MPP down-regulation which I haven't seen in similar design and simulation software so far! that made me love PVSOL even more. Finally, I would be glad to hear your opinion about solving this problem should you don't mind.
Thanks again and all the best