Trying it in a fresh new project does make it work. As I mentioned before, when I disable all of my plugins, this is the message I get. I apologize if I wasn't clear on that. If you like, I can give you a copy of the two plugins that caused an issue in the previous message.
I was able to identify a potential cause for your error. In RPG Maker itself, go to "Game" -> "Update Core Scripts" and choose a newer version. This plugin was designed for version 1.8.0 and newer. That should resolve the error.
We already found out, the actual problem was that OP used an older core version. My plugin will now check for the version of the core code and prompt the player to update it when it's not up to date.