When uploading a multi-part file to S3 (>100MB), the upload fails if the destination path begins with a space.
Looking at the log from the transfer window, it appears that the actual upload works correctly, up until the point where a POST request is sent that excludes the leading space, which gives a 404 error from S3, and the transfer fails.
If you re-attempt to upload the same file to the same location, it fails immediately without uploading anything, presumably because it attempts to resume the existing multi-part upload. In this case it is a GET request that excludes the leading space, leading to the problem.
I've included (redacted) log files demonstrating a successful single-part upload to a path with a leading space, and first and second attempts to perform a multi-part upload.