When I have more than one instance of Nebula on a track, I am very likely to get an EXTREMELY loud digital beep/click when I initialize a second or third instance of the program. Even after the beep, the channel will show a constant 100db on one or both channels. Sometimes it will go away if I save my session, sometimes it mutes the channel until I move one (always random) of the instances of Nebula to a different slot in the plugin chain. It's very inconsistent and very weird but extremely annoying because of how LOUD it is.
does the reverb version solve your issue? I think cubase is loading the plug on a processor which is busy, the same processor used for processing audio. In such case the threaded version solve the issue