The ruby fog library supports a lot of cloud providers. It started as simple gem that got very complex due to to the increasing support. OpenNebula was also supported but, the plugin got a bit stale and its continuity got jeopardized, but now its updated and merged into fog upstream.
It appears that the fog gem does not support OpenNebula 5.8.1+. Is there a plan to support fog with the latest version of OpenNebula? There is an open issue at https://github.com/fog/fog/issues/4025.
The gem code was updated to be compatible with the release. As a reminder, whenever an issue with the gem is detected, it would ideally be reported here. The providers in fog have now been separated into gems for each one.