Hi Ruben,
I chatted about this with @asevcenc in the JAlien channel on MM and with his help I found a workaround. I checked out XRootD in the version 5.4.2-alice1 as development package and then recompiled O2. alien_cp works for me now. But unfortunately the issue is not really understood.
I am also still using an older version of alidist, because of O2 build fails on: xjalienfs - ubuntu 20.04, possible setuptools issue
Cheers,
Ole