I have found a couple of times that if Zerynth does not close properly it does not delete the singletonlock file it creates in ~/.config/Zerynth (in Linux, don’t know where it is in other OSes), and it is necessary to manually delete it. When this happens though there is no message or anything to indicate what has happened, absolutely nothing happens. Zerynth just disappears.
It would be better, imho, when this happens for Zerynth to display a message that it could not create the lockfile, and should it start anyway. If user clicks OK/Yes then delete and recreate the lockfile if required (or just leave it) and start normally. If the user replies Cancel/No then delete the lockfile and close normally.
I only found this out at it has happened to me a few times, and I couldn’t work out why it wouldn’t start. So I tried starting it from a terminal, and it was only then I got the error message that it could not create the lockfile. Manually deleting it solved the problem.