In one image you see the window inside the program (and there are curtains), in the other image you see the final render (and the curtains have disappeared).
WTF
Hi matteo.manzi, perhaps there is movement of the curtains, when you go to the rendering stage they are open, or the material which is transparent, is not woven.
One of these two hypotheses can be. If not, I don’t know what it could be.
Then if you want a tip, you have to give less light, because it is fired on the kitchen that is already white, and give a little contrast and saturation. It’s a suggestion, then it depends on your tastes.
1- it would be better to write in English so that it can be helpful to more people
2- there is no motion, and even if there was, if you render an image the current position of the moving object is exported (it would be better to know the program before answering)
3- the curtains are an internal asset of D5 with their material (i.e. fabric)
4- I don’t need unsolicited and incorrect advice on my work, but thank you very much anyway for your interest
Ok, I was trying to help you, sorry, I have full respect for this job because I love it.
Then I didn’t want to offend your work, but if you see the writing 2 times, give it the chance, if you can help yourself, for me criticisms or advice are always welcome, even in my work they give you a way to improve yourself.
Going back to the problem if you let me, try putting another curtain from the graphics program you use, then with the I key of the material selection, take the fabric of the old curtain and apply it to the new setting by you, then see with the render if the same thing happens. Sorry 
Hi mate, can you tell me the name of this curtain so I can check and reproduce the issue? Also, Can you screenshot the settings here?
Sorry but I had some work to finish and didn’t have time…
I attach image.
The curtain is Curtain 04_Open.
However there are literally thousands of factors that could cause the bug, so I don’t think it can be replicable.
I noticed that it only happens on the right side of the frame.
Even the curtain disappears live the moment I move the view, even without rendering.
I get the impression that the software removes objects out of frame to speed up the final render, but in this case it miscalculates what is really out of view and what is not.
Hi, as you said, we cannot reprouce the issue 
. To confirm, is your version 2.5.1?
Can you keep this curtain and the scenes( you can delete other objects) then send this part to us for troubleshooting? thanks a lot.
upport@d5techs.com