You were overwatering your plants as I told you in your last thread when you said you gave them 6-7 liters each per day in those 19L black plastic enclosed buckets.

Most likely the roots are now drowned, and at those 35 C temperatures and lack of air circulation enhanced by the heat absorbing lack plastic, the oxygen content is low of the media and the wetness and lack of ventilation causing ideal conditions for the proliferation of root rot. Everything adds up. Also the container is small and if you put too much fertilizer in it, it can concentrate until the roots can no longer raise water. But that would be likely a more gradual decline, not as sudden as yours seems to be.

good luck

Thanks for answering me.

I don't think one can overwater a plant in a self watering bucket. There is an overflow pipe at the bottom, and I can't add more than 3 litters at once.
The plant is supposed to drink as much as it needs from the reservoir, by capillarity action.
The roots of the plant don't come in contact with water also.

I did wrap an aluminium foil over the buckets as some of you suggested and it seems to drink less, probably about 4 litters per day.

I also used the fertiliser dose as indicated by Earthbox manual with respect to my bucket volume.

What has been done, has been done. It will be a lesson for the next year.

But, is there anything I can do to save the plant now ?

I'm glad you took the aluminum foil suggestion. Even though they are self-watering containers, there was no barrier to the reservoir around the perimeter of your pot because of the design of the bottom. I think that is a design flaw but in Romania maybe it is popular and works if the humidity is low enough, I don't know.

seysonn also commented the same thing in that thread, as he was concerned they were too moist, so I'm not the only one.

To see what can be done, water only once in the morning and remove everything to keep the top exposed to the air, in hopes it will dry out. Go lighter on the fertilizer, too. Good luck.