Last time this happened on my phone, it happened part way through syncing, and tapping this button on my phone deleted hundreds of my files to force it to finish syncing, as well as pushing the changes of deleted files and folders to my laptop.
Right now I’m very worried as the folder it’s trying to sync to my phone says absolutely no data has been synced yet. I really don’t want to lose the whole directory; is there anything I can to to apprehend whatever bug is causing this problem and make my phone sync?