I had this problem since today. When I open a vstwindow with the ‘Edit VST Instrument’ button. the VSTwindow doesnt appear at all. The only way to fix this seems to be to rightclick Cubase from the taskbar (I use Windows 10) and activate that while having the 'edit vst’button on. After this my VSTS do appear back, but I have to repeat this process each time in this project before I want to edit something, which is quite frustrating.
Also I seem to have this issue only when I open a specific project where I use a certain guitar library in Kontakt 5. The library itself seems to behave strange too, as the sound often changes like there is a huge lowpassfilter in it out of nowhere. After testing in other projects, the vstwindow seems to behave normally.
Is there any way around this?