Too much water, or just as likely, the plant is reacting to its new environment. It was growing in the greenhouse with perfect watering, lots of sun, high humidity, perfect fertilizing and temperature. Now it has none of these, and it has to cope. It already is experiencing lower humidity, and if it is close to a heater, that will make it worse. In addition, Lilies (and all plants) prioritize the health of the flowers over the health of leaves. If a plant cannot support both, it will allow leaves to die so it can support the flower.
Too much water can be part of this overall stress, since when there is too much water, roots do not function properly. However, not enough water does the same thing. More often, when I see lilies grown in pots for sale, they suffer from this overall stress, rather than only too much water.
Even if the stem and flowers die, the underground bulb should be alive. You can plant the dormant bulb outside in the garden to come up in the spring. I don't think the ground freezes in Ljubljana. (I have relatives in Novo Mesto and Metlika.) When the top dies down, stop watering and put the pot in an unheated place like a garage for one week. Then plant outside in the garden.
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