what is going on? any idea?

Well, the yellowing in between the vains with dark green vains is a shortage of iron. So, lockout as said down below would be part of it. So, flush your pot with 3x times the vslue of your pot. And let it sit for 2-3 days, wait till the top soil drys out some and start of with whatever you have that had iron in it, it'll have (fe). After that feed. Try to go lighter on on the nutrirbte or if you have big dif in your water PH and fert mixs it could also be causing issues. So try and keep all feeds near the same PH. as follow up water feeds go on top of your fert feed and if its out of whack you'll be seeing lockout.. Good luck.

I recommend saving this link on your phone or computer for future reference of plant deficiencies and toxicities: https://dutch-passion.com/en/blog/a-visual-guide-to-cannabis-deficiencies-n987

looks like abit too much K and maybe a little lockout possible too. leaves look glossy and overfed in general.. dial back... wait and see. at 6 weeks it's very unlikely a amification of coco's properties (if not buffered well) ... so i'd definitey tweak overall concentration as well as conside a lower ratio of K relative to other components. stat with a 5-10% drop if you can and see how it goes... the existing damage will not heal, but if the progression abates you know you did soemthing helpful. there may be lag betweeen making a change and seeing an effect... as long as it isn't progressing to quickly, be patient in this process... as patient as you are allowed by the plant. it's beneficial to track what you feed in a soilless context... it's going to work in a very consistent way if you keep other variables steady - temps, rh, light, co2 etc... if you see a k-tox at a particular lthreshold often, you can be confident you've found a boundary for K, assuming other stuff isn't shifting aound too much at same time... a lot of this stuff is elative to each other. as one changes, it may require a shift in somethign else. too. manic botanix has an online calculator... i usually don't hand this link out in a public setting but you can download my spreadsheet too.. there are no macros... PSA - be wary of spreadhseets, they are potentially very dangerous... this is s simple spreadhseet with no macros and only formulas in cells. so teh most basic software will work. it as made on apache openoffice calc. https://drive.google.com/file/d/1404ph0LPe3XlMwlfJf5LLLy7xRRM0i7I/view?usp=share_link won't be any different than the online calculator except it will tabulate ppms for you... type in info from guarnateed analysis lables just as you would on teh website calculator... it will add up everything in teh output section (green) automatically for each molecule. there is a comment in cell A1 you cannot see in google docs... this contains instructions. Also, you cannot edit the google doc as linked. i do not give those permissions so you have to download it to use it... or just use manic botanix (googlel search name + nutrient ppm calculator) if you track this stuff and see when problems occu you can make little notes... like i said it is more complicated than that, but over time you will see clear boundaries for each molecule where toxicities inevitably occur... or deficiencies... you can know with cetainty you are pushing boundaies... and this isn't something you need to recalculate constantly.. jsut knowing a baseline is good.. and small adjustments around that baseline will be needed occasionally. . don't need to calculate small adjustments.. you know what 5% more is without doing so. the benefit is speeding up learning curve and reduced trial and error necessary to find a happy zone.