hi Marius, thanks for the awesome work testing this. Should you be able to collect the /tmp/mistral-ansible-action directory which contains the ceph-ansible cmdline and inventory file, that could be helpful.
Created attachment 1450853 [details] ansible-playbook.log
verified on ceph-ansible-3.1.0-0.1.rc9.el7cp.noarch
Since the problem described in this bug report should be resolved in a recent advisory, it has been closed with a resolution of ERRATA. For information on the advisory, and where to find the updated files, follow the link below. If the solution does not work for you, open a new bug report. https://access.redhat.com/errata/RHBA-2018:2819