KDE Plasma with SDDM and Xorg: .Xauthority missing

My system is an Arch Linux, with KDE Plasma, SDDM and Xorg.
I installed Plasma and it came by default with Wayland (or at least that is what it was logging into). I started using Plasma with Xorg later. But then I noticed that x applications were not working, like xclip. I installed xeyes and xclock to test it but every time I ran any of those applications I got

Authorization required, but no authorization protocol specified

Error: Can’t open display: :0

I confirmed that the $DISPLAY variable has the value of :0:

$ echo $DISPLAY

:0

Some posts suggested to run xhost but it got the same issue with xeyes, xclock, xclip and all other x applications: “Autorization required…”. Then I noticed that .Xauthority is missing in my home directory.

This answer even suggested a way to create the .Xauthority, which does create the magic cookie, but I started getting the error:

Invalid MIT-MAGIC-COOKIE-1 key

Error: Can’t open display: :0

Others suggested to just restart Plasma that the .Xauthority is re-created automatically but I already restarted many times and it hasn’t been created.

$XAUTHORITY points to the right place

$ echo $XAUTHORITY

/home/myuser/.Xauthority

Can anyone help?