The event based file watcher that Prepros uses by default can cause issues while watching files located in a network drive so polling file watcher is required to watch a network drive. Please enable polling file watcher in app settings. If you are also having issues with polling file watcher, please check your network speed. You can also try increasing the polling interval to 1000ms or some higher number.
If you have already added a folder that crashes Prepros. Please delete one of the following config folder depending on your OS. That will reset Prepros configurations then you can enable polling file watcher.