It seems the problem could be caused due to some unexpected app crash or computer crash. Most likely it is caused by the sync task list file. Please try to delete the file:
(The path could be different based on your OS type and OS drive)
Note the file is usually locked and you may not be able to delete it. To unlock the file, please open FileManager, click on Tools (menu) --> Unregister Folder Icon in Explorer. Then exit FileManager. (make sure you exit it by clicking on File menu, select Exit. Close the application window does not exit FileManager).
You can then delete the file. After it is deleted, please restart FileManager and it should be fine. The bug will be fixed in the next release.
(If you still cannot delete the file, you can reboot your computer and try again).