Massive root rot on my Phals

[Najsha]

Hi everyone!

I'm noob about orchids but I love to tend them as a hobby of mine. Everything was perfect for some time, I had two orchids which were blooming for six months and they were very healthy and happy. A month before I bought four more from a discount sale. They finished blooming but they had healthy roots and new leaves were slowly emerging.

Everything was perfect, until I decided to repot them. Silly me..
In our stores in Slovenia is very difficult to find a very good substrate (with bigger particles) but I found a great looking coconut chunks. It was a package, prepared specifically for orchids, it required only a soak and repot. I've happily repotted all my orchids.
And then the nightmare begins... All of my orchids started dying very quickly! The roots started getting brown and black and some of the leaves started wrinkling and yellowing. Now I'm devastated and I really don't know what to do. Are they dying beacuse they were planted in a different substrate? I was searching for possible anwsers and I learned that the coconut chunks must be soaked and washed thoroughly to remove excessive salt which is hiding inside... But on chunks package this isn't listed, it just says it's already prepared for use.

What do you proffessionals think, what is killing my plants and how can I rescue them? It's healthy to repot them again?
I'm attaching pics of my sick friends..