By default, Maya loads the Houdini Engine backend into the Maya process, which is probably causing conflicts in Maya 2018. It's also possible to load the Houdini Engine as a separate process, which would avoid library conflicts all together. Normally, you'd select it from the Houdini Engine preferences. But since you aren't able to load the Maya plugin to begin with, you could run the following MEL to set the same preferences:
optionVar -iv "houdiniEngineSessionType" 2 -iv "houdiniEngineSessionPipeCustom" 0;
Then, load the plugin.