Thank you for the hydrobuddy!
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This morning I sprayed the plants with epsom salts (teaspoon per gallon). It was lights out… I didn’t want to burn the leaves in the light… But now I read that when you spray in the dark the stomata will be closed… :-/
It seems like the yellowing goes from the lower leaves to the more upward leaves… I dont like that kind of progression…
So it can only be magnesium or nitrogen deficiency…I guess 
What can I expect to see with foliar feeding if there was a deficiency? Will yellow leaves go green again or just green leaves will not turn yellow?
I am a bit unsure how to proceed now…
If I have salt build up or lockouts, can I just change the res or do I need to flush with plain water? I have not enough plain water available to flush…(But I have flora kleen around?)
Can I addtionally foliar feed half the nutrient strength of my nutrients everyday, so the plants will be independent of the issue in the medium?
Btw how can I get a Nitrogen deficiency with lucas formula in 3rd week of flower? What would lock Nitrogen out?
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Your pH is out of ballance, is the most likely answer to that question.
Change your res, lower your pH to 5.5 and see if they won’t peak up.
Maybe post a picture, would make it a lot easier if we could see the plants.
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He’s been asked on several occasions to upload a photo but he’s afraid his personal data will be available.
If he doesn’t trust how the site works he can run his photos through an app to remove the sensitive information before hand
That will do the job 
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Yeah a picture is worth a thousand words in this situation. SO much depends on guess work otherwise… there could be any number of things… the extent of yellow leaves being one… It’s not at all uncommon for some of the lower leave to yellow out even early in flower, as mine get crowed the lower ones get less light and they yellow for example. It’s could be no big deal … you might be adding this and that for no reason at all for example… you could even be causing problems for yourself…
@Palindrome has the good advice, dump the res, start fresh and set the pH as recommended. Give those roots a good dose of mychoriza… And most of all Don’t Panic 
The thing to remember is that tap water with the lucas formula and nothing else has been used for countless DWC grows, it is reliable and deficiencies should be exceptionally rare. You really shouldn’t need to be adding additional nutrients.
When I have had issues it’s always b/c my res was too small and I got lazy with change over or I allowed pH get out of hand, so it pays t o make sure you pH tester is calibrated properly and regularly.
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I try send a picture soon. But I am very busy right now.
I think it main reason is probably temperature drop. Its getting too cold outside for running with external air. Plants stalled. Couple of hours in early morning when lights off (4 am) plants get very low temperatures. Last night outside was 3 degree celsius… When lights on max is now 20 degree celsius.
So I need fix temperature.
Thing is I can do it sealed but my dehumidifier has trouble.
I have running four ebb and flood tables with total 100 to 150 rockwool cubes (2,5 inch).
I think open tables with that amount of cubes and the flooding release too much humidity into the air.
My thinking is that when I can put a cover on the tables a lot humdity will stay there.
Today I tried strech foil. But it didn’t work. It not sticked to the edges, also when cutting holes into it to put the cubes through, the foil shrinked too much instead of covering the cubes.
I am thinking now about covering the tables with aluminium foil. Maybe that is rigid enough to form it, and easy to set up and to put the plants through it.
Or maybe something like a rescue blanket, or a truck canvas???
Can you recommend me a material for ebb and flood tables as cover with many small plants, which is easy to set up?
White trash bags and some sticky tape.
A sheet of painters disposable 'drop sheet with ‘x’ holes cut for plant stems.
Panda film and pegs or clamps.
Polycarbonate with holes drilled or strips that sit across the tables between the plants.
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Is it worth buy panda film for such endeavour? I never used pandafilm so far and dont have it around. I think pandafilm is sticky on one side? Will the sticky side cause me problems?
I was thinking about the polycarbonate but still I have not dialed in the final plant count for the tables.
So I need a foil or canvas that covers enough area and doesn’t crumble when i cut an X into it.
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Panda film is not sticky but black on one side and white the other.
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I will try garden tarpaulin and rescue blanket. Both has size enough to cover the full table. Especially the tarpaulin has a mesh enwrought. Maybe that makes it easy to cut the many holes.
Also I did find some clamps for tablecloth…
I will see how that goes.