This is a bug in the plugin that happens when multiple instances of the plugin are involved. Loading it with only one audio CPU in Renoise will work fine, but that’s not a fix.
A quick look at their code shows that there’s at least one problem here. They are using static variables to initialize things in audio time - simultaneously:
You can’t use static variables in that context. This code can run asynchronously in audio time from multiple threads at the same time. So multiple plugin instances will be using the same temporary state at the same time. This will break sooner or later.
And I appreciate the time you spend writing such reports for Renoise, but please send them to the plugin developers first. If there’s a bug in Renoise, they’ll let us know and we’ll be happy to fix it. But we can’t do the support and debug all the thousands of plugins out there.