RC[1,2] seems to have issues with some build actions if compute ressources are involved. The problem does not occur with "bare metal" hosts.

Setting up a new host...
- "new host"
- Deploy on: oVirt
...works just fine. A VM is created and provisioning starts as usual. Only a few actions result in failure:

A) calling /unattended/built
B) click "Cancel Build"

The error message is basically the same:

A)
Failed to set Build on testhost001.install.example.com: undefined method `symbolize_keys' for nil:NilClass
Completed 409 Conflict in 192.7ms (ActiveRecord: 8.6ms)

B)
Failed to set Build on testhost001.install.example.com: undefined method `symbolize_keys' for nil:NilClass
Failed to save:
Redirected to https://fw.example.com/hosts/new
Completed 302 Found in 81.2ms (ActiveRecord: 8.7ms)

Attached you'll find a full debug log. Unfortunately there is no stacktrace available, although "debug" logging was enabled.