18.0.0.0 is okay as the plugin uses this API also for VEGAS Pro 19 API.
I have to admit I'm fighting this bug for years now, and honestly I don't have a clue. It seems it's not in the code, I assumed it was something in the metadata / resources of the plugin dll, but the VEGAS Pro guys said i was wrong with this ... so this is still unknown why ít sometimes doesn't work. Sorry I don't have better news.