Hey,
I tested a little bit and found out, that the last working version of Magisk on this ROM is
Magisk-v18.1 (assuming we're on v12 without Aroma package and on allucard kernel edit: mine is a GT-I9505 (jfltexx) btw).
When installing any higher version (19.0-19.3, tested all of them), the ROM boots indeed but after unlocking the lockscreen the launcher and every other app crashloops and you get spammed by crash notifications. Trying to reboot will get you stuck in a bootloop.
I don't know anything about android and kernels but i guess it's some compatibility issue since this rom and the kernel are a little bit older.
Just flash v18.1 via twrp and never update. This rom is in EOL state, therefore this could only be fixed by the Magisk developers. Maybe some later version will work again...but most likely not.
Something else...
Installing the Aroma package kinda doesn't work for me. Obviously just trying to flash the .zip from github didn't work. I tried to combine the archives by replacing the files from the rom zip with the files from the aroma zip and flash it. I got into the aroma installer assistant but when it's about to install it just goes to the installation finished screen without actually doing anything. Anybody got a trick for that?
@black.kahder you said you got it working somehow. Would you mind explaining to me what you did?