what could have happened?

Lot of good answers below, Have the agree it is Calcium deficiency. Get some calmg in there and Link your diary. Happy Growing!

Also stay with veg ph until they start to bud.

5.8 through veg and 6.2 to 6.3 for flower. When you first switch to 12/12 you should stay with your veg nutes for the first week of stretch, then start giving some bloom feed with your grow. When you see actual flower is when you should switch to mostly bloom nutes and a little grow mixed in for the next 5 to 6 weeks. They will need their grow formula for the vigorous stretch the next 2 weeks.

Today i switched the light to 12/12 and i added fertilizers for the flower. I measured the pH value in coco again and it came out again 5.9. So i must use a calmag you mean?

You are having calcium deficiency. Coco is hydro in a bag or potter. So your ph in vegging going in, and comming out the bottom in run off should be 5.8. Then 6.2, 6.3 in flowering. Coco always requires a cal mag supplement. I grow in coco daily. I use 1ml to liter in my Coco grows. Your nutrition level will also control your ph run off. Coco needs to always be wet. You can not over water in coco. Heavy. Heavy, irrigation is required in coco for really strong growth. Feel free to look at any of my auto grows in last 9 months. I can almost put money on you have a ph issue as well in coco. Will also cause the cal-mag lock out. Lots of experience with coco grows.

That's a calcium defeciency my man. Either add some calmag to your feed or double check your ph. Another nutrients could be blocking calcium because of salt build up or incorrect ph rangehttps://www.420magazine.com/community/threads/which-ph-chart-is-true-for-coco-mainly.443096/

AN - grow, micro, bloom and b52

Nitrogen deficiency

looks like Nitrogen to me, try a bit more vegetate fertilzier asp idk what nutes u running