I have the samew issue with my windowsa PC. I used SYNC folder when in linrary mode to pickup missing pictures. No matter what I did the system would freeze. I was told to do a Import instead of sync for adding new pictues?
'Sync Settings' and 'Sync Folders' are different. So, your issue is not the same as that of the OP. I suggest you start a new thread, and while doing so - provide details of computer, operating system, version of Lightroom you're using, etc.