I'm having similar trouble with all VSTs exported from Cabbage in FL Studio 12.4 on Windows 10 with Csound 6.10; the VST GUI is blank (just a black rectangle) and doesn't seem to respond to MIDI input or otherwise generate or pass through sound. Hopefully I'm not reviving too old a thread. The standalone Cabbage program works completely fine as far as I can tell. I'd be happy to try building from source on my system and trying some debugging if that would be helpful; I'm not super-knowledgeable about C++/JUCE or the Windows platform but might know my way around well enough to be of some use.
I don't see a "Used fixed size buffers" option in my version of FL Studio, but looking at the ASIO config it looks like the buffer size is fixed at 512 samples.
If we could get this to work I'd be overjoyed! Csound has some wildly powerful opcodes even compared to big expensive commercial synths, and I've written instruments in it I'm really fond of, so I'd be super sad to have to give them up in order to work with this nice sequencer.
EDIT: Oh, almost forgot to mention, I'm using Cabbage (64-bit) v2.0.02. As a side note, I assume the exported VSTs are 64-bit as well?