In case you're getting an Error 400 when trying to upload to Amazon S3, you've most probably selected the wrong region for your S3 bucket in the Dropshare connection preferences.
Prior to version 3.8 (Mac), us-west-1 and us-west-2 have been mixed up which might cause this issue. Please try to select a different region in the Dropshare connection preferences to solve this issue.
Feel free to contact the support at anytime in case this does not help you resolve your issue. Make sure to include log files.