[ovirt-users] Cannot update oVirt Node

Hello, I am trying to update my oVirt installation to 4.5.5, but while I had no problems upgrading the self-hosted ovirt-engine, I am not able to upgrade any node (running oVirt Node NG 4.5.4 or older). I just click Upgrade in the oVirt Manager, and I get a bunch of events telling me that everything was OK and the upgrade succeeded, but in reality nothing happened on the nodes: the result of "nodectl info" before and after the upgrade process is exactly the same. I gave a look to the engine.log and to the relevant ovirt-host-mgmt-ansible log in the ovirt-engine VM, but I cannot find the problem. Do any of you have any ideas ? I am attaching some relevant files. Note that the upgrade consistently fails in the same way on other nodes, which have other versions of oVirt Node NG (always in the 4.5 family). Thanks for your help --gianluca