TLDR: Citrus keeps turning black and oozing resin. But I can't find the problem. I thought it must be root rot, but they look perfectly healthy:

Long version:
Because my first and second citrus trees fell victim to root rot, I started using a very airy substrate made of pine bark, perlite and some humus/worm castings in a 5:1:1 ratio for all my plants (figs, pineapple, cherimoya, monstera, etc.) with little adjustment. You may recognize this as 'aroid mix'. But it works surprisingly well in my indoor space with a west-facing window and terracotta planters (and my tendency to overwater).
But I can't wrap my head around citrus.
It always starts with rapid growth, followed by very suddenly dropping and crisping leaves, black stems and finally death.
I thought I must be root rot again, which I need to mind during winter. But today, when I dumped my fifth (!) tree, I found only perfectly healthy roots and nice smelling substrate.
I think it must be a pathogen… but what?
I am at a loss. I keep killing my citrus trees and I don't know why. :'(
EDIT: replaced "5:1:1 mix" with "5:1:1 ratio" for clearification.
Once a user came into our office on the verge of tears. Her notebook wouldn't boot and she thought that meant her thesis was lost.
Didn't make a backup either.
But luckily it was the mainboard that quit and not the SSD. So we were able to decrypt it and get her up and running again. After we told her to make a backup next time, she was so happy that she wanted to give us money. We refused.
Come next day, she stormed in, without saying a word. Just threw a pile of candy and a handful of soft drinks on our table and ran off before we could do anything about it.
Fuck you, boss. That's our candy now.