I used to have the same problem. Luckily it doesn’t crash anymore. I think if you update Sylenth to the very latest release it may work… but I’m not exactly sure what I did, maybe I also did fiddle with the VSTi settings, here’s some of mine and a link to my old thread about it:
Hive - Checked can run in multiprocessor enviroment
Spire 1.1 - Checked use static processeing buffers
Sylenth - Unchecked everything
https://forum.renoise.com/t/osx-unreproducible-crash-ana-synthlen-hive-vst/45288