Hi just just encountered the same error in my lap,

complete new NFS server new ovirt node installation and first deployment of Engine

[ INFO ] TASK [ovirt.hosted_engine_setup : Activate storage domain]
[ ERROR ] Error: Fault reason is "Operation Failed". Fault detail is "[]". HTTP response code is 400.
[ ERROR ] fatal: [localhost]: FAILED! => {"changed": false, "msg": "Fault reason is \"Operation Failed\". Fault detail is \"[]\". HTTP response code is 400."} 

My logs says the same thing