A further investigation led me to the conclusion that the memory pointing to the in buffer is the same as the out buffer.
I am now successfully processing the in buffer (good, as it would be equivalent to writing code thinking true means false) and found the source of the problem was a memset operation where I would clear the out-buffers by setting the values of the float** arrays to 0. This had the result that the in-buffers would also be empty (I tripled checked whether I was operating on the right pointer! Though I would have found out sooner if it weren’t for the fact that Bitwig Studio and the editorhost worked just fine for the VST3 version…)
I don’t need to use a memset operation to clear the output buffers as I will overwrite them sample by sample in my process cycle. However, is it common to expect that the in- and out-buffers point to the same thing ? Is this related to VST version, VST category type, the host, etc ?