Zkittlez in the Sanlight

Day 52. It has been a rough week. I noticed some leaves turning very yellow, it looks like Mg deficiency. So I increased BioGrow and gave some Cal-Mag. But the following days it became even worse: Many leaves showing ugly brown spots, they appeared almost overnight and spread very quickly. I have no idea what it is, it pretty much reminds me of the problems in week 3 with this lady. I do not think the problem is the absence of needed nutrients, but rather the plant's inability to uptake them. Maybe I overwatered her and the suffocated roots could not do their job. Or this strain is just very sensitive to something I did not yet identify. I did not water her for three days now and only gave a smaller amount of water today. Hope she recovers once again. Maybe it was too much Mg which locked out Cal? So many questions...