@tobitdsm There's quite a lot going on here which raises suspicion regarding your setup and workflow.
First the workflow in that video is a touch out dated, but I believe for all intents and purposes it should work. Just create the C++ node, write your code then Compile the DSP networks as DLL. From there you can load the C++ node in an FX Slot or inside of Scriptnode and attach the parameters to your UI.
Secondly, do any HISE based plugins work for you inside of Reaper or any other DAW? Say for example if you attach your one knob to a gain node, does it function properly? It could have something to do with your setup.
Third, easiest way to have someone help you debug this would be to share the C++ script or a very minimal version of it. This way someone here could rebuild the plugin on their end and test it.
Fourth point you can disregard if you feel like it, but FFT is a pretty complex topic. An external C++ is definitely the way to go but I would recommend starting with something simpler if you are just beginning.
Last point is to make sure to get rid of the old plugin binary versions in your VST3/AU folder. Use Export>Clean build directory before compiling the plugin in order to clear your build folder. Also maybe try changing the plugin code in the project settings.
