The mod uploader can't access to directory whose name include non-ASCII character. This includes creating temp. files & temp. directory, along with the OS username. My occasion is about Windows 10.
Edit: This bug is very critical, as it seems like the failure to creating temp directory results in attempt to delete whole files in its root directory (practically not, as the deletion also fails).
Try uploading mod by Don't Starve Mod Uploader in OS profile whose username includes non-ASCII character (such as hangul syllables)