Hello.
Because of the lack of space I was forced to move some of my plants to the bathroom. I know it sounds silly but that was the last space I had.

So the problem is that all the new growth dries off and falls in the first days and some of the old leaves fall of just like that. The baby fruits also fall.

I have directly under that shelf a radiator and next to the plants a weak fan (bad thing I know!).

The temperature in the room is around 85F+/-2degrees (30C).
The humidity in the room is very high (more than 80%).
The light neon and bulb are plain cheap supermarket ones.

And a question: did anyone manage to grow a lemon tree in artificial light conditions only?

I have many plants that i have isolated from the sun and under artificial lights, my lights are not plain neons but "gro-lite" brand bulbs meant for plants and aquarium pets. For a while they grew well but became a little top heavy and spindly due to lack of air movement, therefore those that have gotten the first signs of being top heavy i've put outside for the summers and in for the winters, and that has helped most toughen up at least for me.

as to your problem i'd think lack of light, and perhaps too much radiant heat causing the leaves to dry out and drop as well.

hope my own trials help you out at all.