Spots on leaves

Please offer any advice.

Indoor
Soil: Biobizz All Mix
Nutrients: Biogrow, Bioheaven, Biobloom, Topmax
Runoff PH 6.6-6.7
They are 2 weeks in flower. Yesterday (day 16th flower) one of them started show these symptoms in a few of the upper leaves (one node below the main colas)

The 2 first photos are with HPS off....




WTF is wrong?
The same plant LINK-->had shown similar symptoms (i think) after the transplant in the final pots due to sudden ph change. The problem had stopped there but yesterday suddenly these new spots in the pictures above have shown.
I have 4 plants from the same strain (nl x skunk#1) treated the same way and the only problems I have experienced is these goddamn spots with this particular plant.
The only difference I can tell is that is the closest plant to the fan and the door of the closet.


What's the matter? Are the two incidents connected with each other, or am I facing a different problem this time? Ph seems to be ok...
Shall I use some Epsom Salts?

looks like iron or cal/mag lockout.

I water every 3-4 days, feeding with nutes every second watering. So probably I am not overfeeding them at all. The ph of water is 6-6.1.
The last watering before spots showed (4 days before spots) had 1 ml/l Bio-grow, 1 ml/l Bio-heaven, 1 ml/l Bio-bloom and 0.5 ml/l top max with ph of 5.9-6.
I haven't measured tds/ec but I know from a friend that this dosage of these nutes is supposed to be about 600 ppm.
Today is 17th day 12/12 and yesterday (after I show the spots) I watered with plain water (half deionized/half tap water).

Allmix is supposed to have 2,4 EC so according to 1) HANNA TDS 2.4EC=ppm1200+600 nutes bio=1800ppm
2) TRUNCHEON TDS 2,4EC=ppm1680+600 nutes bio=2280ppm


Aren't these calculations correct?
What do you suggest according to these infos?
What should I do?

One plant out of four displays the problem, the rest of them seem to be ok.

Any suggestions? Anyone? As far as I can see, the problem is pretty similar to the photo D in the picture below....
Shall I foliar feed with Epsom Salts or it is better to water the soil with Epsom Salts???