What plugin version are you using and what drivers version do you have installed in your system? We don't have other reports about these kinds of critical issues with the latest 2018.1 Octane builds, so perhaps the problem is related to your system configuration.
About the volumes, be aware that the volumetrics rendering has been improved in Octane 2018.1, so the rendering can be not the same than in Octane 4, but you have the option "Old Volume Behavior" in the kernel nodes to render the old scenes using Octane 2018.1 without changes in the volume rendering.
Hope it helps,
Computer Graphics Software Developer
Octane plugin development and support: LightWave and Houdini