I had the same problem for the last 3 days and just figured it out, Sylenth1 constantly kept crashing my friends ableton in important projects we needed and we had no clue what to do
we spent the last 2/3 days googling everything we could, there are actually numerous threads on this site about this but not a single one has the answer
we tried every version of of Sylenth1 and every version of ableton we had stored and it made no difference...it was versions it worked on when we worked on it years ago but wasnt working now
but we figured it out about 10 minutes ago.. it was our anti-virus, we switched to AVG a few months ago. i randomly just decided to uninstall it (just being desperate) and sylenth1 started working again. i decided to go back to my old anti-virus (Avast) and everything is back to working
my guess is some antivirus programs flag or block some part of sylenth1 from working properly
i hope that helps

maybe it will spare someone some wasted days trying to figure it out