Thank you for your input, guys.
Picked up the plants out from the pots (carefully, tried not to destroy the roots) and mixed the soil with a different soil: high peat - with addition of coconout, perlite and barks, this new soil that was added to existing soil has little or no fertilizer in it...
I water them only when the soil is dry. Only. So I think that over watering is not the cause.
Do you think too high pH can be the cause? The ground is a mix of peats enriched with Osmocte WE NPK 15-09-09 and starter WE NPK 10-52-10 fertilizers, with addition of ceramsite (which increases pH of...
Every leaf has strange, easily visible light-green colored 'stripes':
The plant doesn't grow at all. Leafs are directed to the ground. The second plant has these strange light-green 'stripes' too but manages to handle that defficiency or a disease or whatever it is a little bit better, but...