Sorry to hear about it 😔
They just started to get better and now this. I spent some time looking around if there's something similar to this online and found pretty much nothing.
The best I can tell is that it might be dampening off, on your "3 September. Time to say bye" picture the stem appears to be completely brown for 2-3cm off the ground.
To me, this makes no sense since you have good fabric pots and lots of perlite that should in theory provide more than enough oxygen and drainage.
Don't get discouraged, even seasoned gardeners started with wilted leaves and yellowing plants. It's all part of the learning process.
What helped me get the guesswork out of watering is the TFA Dostmann Combitester, which is usually around 10-15€ and well worth it.
Hope to see more diaries in the future!
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