Windows client needs restart to finish sync

I have a strange issue with my Windows desktop app. If I copy a larger amount of files into my NC folder (tested repeatedly with 4000 files and dirs) the client starts to synchronize and shows 2500 files to sync. It synces them ok and then says All is in sync. I need to click Sync now and then it synces another 1200 files. Then nothing happens when I click Sync now. I need to restart the client and it synces the rest 200 files or so.
There is nothing in the server log. The nextcloudsync log on the client is always empty. If someone has an idea what the problem could be, let me know.