Cymbidium: new growths rot

[Annaswe]

Hi everyone!
First I want to apologize for any misspelled words since english is not my main language.

So, the problem then.. I had a no id cymbidium, bougt it with flowers and everything was fine until i repotted it. Then all the new growths start to rot from the inside (black, soft, non smelly) and eventually, even tho I cut away the infected areas and hydrogen peroxide the cut, it spread and the orchid died. So i bought a new one and (almost) the same thing is start to happening
A new growth has a black "hole" in the side of the base. I have clean it and hydrogen peroxide it. Why is this happening and what am I doing wrong? This cym has its own fan now and sits in a South facing window. Since i live in Sweden the sun wont burn it and it cant live outside atm cuz only bears and mooses can tolerate this shit weather...
The medium is a mix of medium bark, some spagh and perlite. When i repotted it, i hydrogen peroxide the roots and the roots were in good condition and almost non were damage. All of the old medium was thrown away.

Hoping that maybe one of You all know what im doing wrong, cuz im not ready to give up on cymbidiums yet <3

/anna