If you have a folder "Watched" that is an UNC path and the connection is lost because the file server was rebooted or crashed. The folder is no longer watched. I have been using Qlik Data Transfer - October 2020 on a windows 2016 server with Qlik Sense. I have to restart the Qlik Data Transfer service and upload the files manually one time, before it is "watched" again.
Regards,
Anders