Hi there @idoki,
I wondering if you are chasing your tail, I tried to check the dns enteries which your two domains control panel would be set up
https://www.whatsmydns.net/#A/fel.lu
gives an ip address of 77.244.243.51 which does take you to an easyname parked domain message.
https://learn.getgrav.org/17/advanced/change-site-url
you can check the system.yaml
but see it forwards to a sub directory of /lb
so you might find there is a redirect to the domain running as well as the fleegeelteren-lëtzebuerg.lu doesnt report a ip address.
Maybe just do a test and install a fresh copy of grav in to a sub directory, lets say grav-test and then see if you can get access to the webspace by using the ip and the subdirectory in the browser by http:\\77.244.243.51\grav-test\
to ensure your host (easyspace) is accessible, is something weird happens, you know it either the system.yaml or a .htaccess or redirect or custom route that has been setup
Just to add, i thought I would ping the address of the working website and got this link https://xn--fleegeelteren-ltzebuerg-2dc.lu/lb notice the prefix xn-- , so some sort of redirect in place
:) which traced to the visible site being on https://kasserver.com/