By default in Mabox picom is running without config (picom.conf is symlinked to /dev/null), so it uses built-in values (well tested and should work everywhere).
It is worth trying the provided modern.conf file in which properties such as rounded corners and background blur are enabled.
hI @napcok, thank your for your fast and very helpful reply. I was so focused to solve this via terminal that I did not realize that the ui of mabox has the solution for me.