Well unfortunately I play mostly in the dirt and ppms mean nothing to me lol... but...
1) Your temps are very low to start with. Frequently excessive cold will combine with another factor to make lockouts even worse... roots unable to uptake nutrients efficiently... all chemical reactions are temperature dependent, remember; cannabis appreciates its roots being at least 65 degrees. Your range is 10 degrees too low; 65 night and 75 day is acceptable. Furthermore, if you are growing in a cellar on a concrete floor, do something to get the pots UP off the floor, like a slab of 1" styrofoam insulation.... that's another thing, in soil anyway it is nice to have the roots just a touch warmer than the air.
Okay. That's the cold part.
2- The symptoms are weird. You say you are using CalMag. Those very clearly-defined stripes on the leaves are symptomatic of Mg deficiency or lockout. Again, temps combined with an acid environment (5.4 counts as an acid env!) CAN make lockouts worse... but I'm not sure about that one specifically.
On the other hand, you might also have the typical N deficiency at the end of flowering coming into play (early it seems in your case). And as someone pointed out, Iron deficency could certainly be playing a role here.
I hope someone with better water experience will come along and have a look.
The problem is that you are far enough along in flower that the usual quick-fix, foliar feeding, is probably not the best idea.
If you had only the very start of buds forming, I'd say go get yourself a bottle of IronGreen (I think they have it at home deopt as lawn food) and try a foliar feeding with that. It's iron plus nitrogen.
I'd be careful adding too much N during flower though.
The other thing is a foliar feed with epsom salts to correct possible Mg def.
The reason I'm suggesting to foilar feed to correct a deficiency is that obviously you are having uptake problems in the roots zone for it to have gotten that bad. BUT you have bud mass already.... yikes... a week earlier, and it would have been a lot easier to deal with!
You should wait for Latewood to drop by but in the meantime get those temps UP to the 65-75 range!!!!