Edit:
Apparently the problem was the RPI OS, that for first, I chose the latest Raspbian 64bit, then used KIAUH to install all components. Today I decided that for the troubleshooting, I am out of options, and the best thing would be, apart from completely giving up, is to start again from scratch. But now, I took the lazy way of doing things, and reinstalled RPI using MainsailOS, 32bit version. I chose that because of some comments I read elsewhere, stating that this is still a somewhat safer way of doing things.
And this worked, I managed to follow the guide without any hiccups, and now I have a running Klipper system, that is able to communicate with both mcu’s through CANbus. I don’t know what the cause of my previous issue was, but it is not important anymore, either… 
Thank you for everyone here for the help, time and effort!