I was depending on a Maven module (actually, two of them) The rest of my project is built with Maven so it seemed logical and looked like it would work. I didn't remember the build seizing up when I added them, so I had forgotten about it. Removing the dependency, starting a debug session, uninstalling the plugin, rinse, repeat and I eventually stopped seeing error messages.
I've since stuffed the maven modules into a jar-with-dependencies
and told the IntelliJ plugin module to load the resulting .jar
No problems so far - Yay!